fix(zen): guard against empty tool_result content in Anthropic provider

[zerone0x]

Summary

Fixes #15371

• Guard against empty/null/undefined tool content in the toAnthropicRequest function
• When a tool call returns an empty string (""), null, or undefined, the Anthropic API rejects the request with "content field is empty" error
• Normalize empty tool content to "(no output)" before building the tool_result content block

Changes

• packages/console/app/src/routes/zen/util/provider/anthropic.ts: Added content normalization in the role === "tool" branch of toAnthropicRequest — empty strings, null, undefined, and empty arrays now fall back to "(no output)" while non-empty strings and non-empty arrays pass through unchanged

Test plan

• Configure a tool that returns an empty string (e.g. a write-only tool or command with no stdout)
• Verify the Anthropic API no longer rejects the request with "content field is empty"
• Verify tools with normal output still work correctly

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[github-actions]

The following comment was made by an LLM, it may be inaccurate:

Based on my search, I found one potentially related PR:

PR #14586: fix: filter empty content blocks for Bedrock provider
#14586

Why it might be related: This PR addresses a similar issue of filtering empty content blocks, but for the Bedrock provider instead of Anthropic. Both involve handling empty content that causes API errors.

The other PRs found (#8497, #8958, #11572, #11882, #12634, #14637) are related to provider compatibility and message handling but address different issues (thinking blocks, reasoning content, tool_use blocks).

However, if PR #14586 is already merged and handles a similar pattern, it might provide useful context for this Anthropic fix.

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