Support FEP-f228 activity collection backfill

[sij411]

Summary

This PR adds the next FEP-f228 context collection path for @fedify/backfill: context collections whose items are Activity objects.

The new strategy handling keeps BackfillItem.object focused on conversation objects. When a supported Create activity is found, backfill extracts and yields the activity's object rather than yielding the activity itself.

Related to #275

Included

• Add context-auto, context-objects, and context-activities strategy handling
• Make context-auto the default classifier for direct objects and supported activities
• Extract embedded or dereferenced Create.object values from activity collections
• Reuse existing request budgets, abort handling, interval handling, and dedupe behavior
• Add tests for explicit, automatic, empty, overlapping, duplicate, embedded, URL, and budget cases
• Document the strategy behavior in @fedify/backfill

Validation

• deno task -f @fedify/backfill check
• deno task -f @fedify/backfill test
• pnpm --filter @fedify/backfill test

AI assistance disclosed in commit message with Assisted-by: Codex:gpt-5.

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📝 Walkthrough

Walkthrough

The backfill package extends its traversal capability from a single hardcoded strategy to a configurable, multi-strategy system. The change introduces three strategies (context-objects, context-activities, context-auto) as a type union, refactors the core logic into composable generators that handle strategy selection and Create activity unwrapping, and adds comprehensive test coverage validating all strategy combinations and edge cases.

Changes

Multi-Strategy Backfill System

Layer / File(s)	Summary
Type system and contracts packages/backfill/src/types.ts	BackfillStrategy expands from "context-posts" to a three-literal union: "context-objects", "context-activities", "context-auto". BackfillOptions gains an optional strategies array property with default ["context-auto"].
Core backfill refactoring and strategy-driven logic packages/backfill/src/backfill.ts	Backfill normalizes input strategies and refactors per-item processing via new async generators: getBackfillItems yields context-objects and unwraps Create activities; getCollectionItems and loadCollectionItemDocument handle document loading; getCreateActivityObject extracts Create objects with budget/abort handling; normalizeStrategies normalizes and deduplicates strategy lists. Main loop deduplicates by object ID and enforces maxItems.
Comprehensive strategy and edge case test coverage packages/backfill/src/backfill.test.ts	Tests cover context-objects behavior, context-auto unwrapping of Create, context-activities collection/dereferencing, combined strategies, deduplication by object ID, empty strategies (no dereferencing), strategy precedence, unsupported activity types (Announce), missing objects, maxRequests limits, and maxItems caps.
Strategy documentation and user guidance packages/backfill/README.md	Documents the default context-auto strategy and adds a context-activities section with TypeScript example, explaining how collection items and Create activities are traversed and that the activity's object is yielded rather than the activity itself.

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~25 minutes

Possibly related PRs

• fedify-dev/fedify#779: Introduced the initial @fedify/backfill package; this PR extends its traversal strategy system and adds configurable multi-strategy support.

Suggested labels

component/federation, component/collections, type/documentation

Suggested reviewers

• dahlia

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and concisely describes the main change: adding FEP-f228 activity collection backfill support to the backfill package.
Description check	✅ Passed	The description is directly related to the changeset, detailing the new strategies, behavior changes, included work, and validation steps performed.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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[gemini-code-assist]

Code Review

This pull request introduces multiple backfill strategies (context-objects, context-activities, and context-auto) to the backfill utility, replacing the previous single context-posts strategy. The default strategy is now context-auto, which automatically handles both plain objects and embedded Create activities. The changes also include comprehensive unit tests for the new strategies and updated documentation in the README. I have no feedback to provide as there are no review comments.

Important

The consumer version of Gemini Code Assist on GitHub is being sunset. Starting June 18, 2026, new organization installations will be blocked, and all code review activity will officially cease on July 17, 2026.
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[codecov]

Codecov Report

❌ Patch coverage is 96.49123% with 4 lines in your changes missing coverage. Please review.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
packages/backfill/src/backfill.ts	96.49%	1 Missing and 3 partials ⚠️

Files with missing lines	Coverage Δ
packages/backfill/src/backfill.ts	84.89% <96.49%> (+5.98%)	⬆️

... and 1 file with indirect coverage changes

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[sij411]

@gemini review

[gemini-code-assist]

Code Review

This pull request introduces new backfill strategies (context-objects, context-activities, and context-auto) to the @fedify/backfill package, replacing the previous context-posts strategy. It updates the documentation, types, and implementation to support these strategies, with context-auto acting as the default. Extensive unit tests are added to cover the new behaviors, deduplication, and request limits. The reviewer feedback highlights that removing context-posts is a breaking change. They suggest keeping context-posts as a deprecated alias of context-objects in the BackfillStrategy type and updating normalizeStrategies to map context-posts to context-objects while ensuring null-safety.

Important

The consumer version of Gemini Code Assist on GitHub is being sunset. Starting June 18, 2026, new organization installations will be blocked, and all code review activity will officially cease on July 17, 2026.
For more details on the timeline and next steps, please review the Help Documentation.

[sij411]

@gemini review

[gemini-code-assist]

Code Review

This pull request introduces configurable backfill strategies (context-objects, context-activities, and context-auto) to the backfill package, replacing the previous hardcoded context-posts strategy. The default strategy is now context-auto, which automatically handles both activities and objects. The changes include updating the backfill function and options to support these strategies, adding helper functions for processing activities and loading documents, updating the README documentation, and adding comprehensive unit tests covering the new strategies and their limits. No review comments were provided, and I have no feedback to offer.

Important

The consumer version of Gemini Code Assist on GitHub is being sunset. Starting June 18, 2026, new organization installations will be blocked, and all code review activity will officially cease on July 17, 2026.
For more details on the timeline and next steps, please review the Help Documentation.

[sij411]

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[dahlia]

It's trivial, but I think the Assisted-by trailer on your commit should be:

Assisted-by: Codex:gpt-5.5

… instead of:

Assisted-by: Codex:gpt-5