feat: add Sailbox sandbox provider

[nirvikbaruah]

Summary

Sailboxes are cost-efficient, long-lived sandboxes built by Sail Research. They are specifically designed for long-running background agents. A SAIL_API_KEY can be created through the platform.

This PR adds a Sailbox-backed sandbox provider under agents.extensions.sandbox.sailbox, following the existing hosted sandbox extension pattern. The provider is exposed through the optional sailbox extra and uses the sail-sdk package to create, resume, execute in, and clean up Sailboxes.

Supported features

• SailboxSandboxClient / SailboxSandboxClientOptions for creating Sailboxes from built-in or custom Sail images.
• SailboxSandboxSession / SailboxSandboxSessionState for serialized run-state resume.
• Command execution through the Sailbox exec API, including concurrent exec support.
• Workspace read/write through Sailbox file APIs.
• Tar-based persist_workspace / hydrate_workspace support.
• Exposed-port resolution through Sailbox listeners.
• Optional pause-on-exit behavior for reconnecting to an existing Sailbox.
• Lazy optional exports from agents.extensions.sandbox so users without the extra can still import the package.

Testing

• Added 67 unit tests that match the coverage of other sandbox providers.uv run python -m pytest tests/extensions/sandbox/test_sailbox.py shows 67/67 passed.
• We ran all the newly created SDK functions on Sailboxes and confirmed they worked: create/start, file write/read, exec, concurrent exec, workspace persist, pause, resume, verify workspace state, terminate.
• Ran an e2e test using the new agents SDK -examples/sandbox/extensions/sailbox_runner.py --model gpt-5.5: agent inspected README.md, handoff.md, and todo.md through the Sailbox sandbox and completed successfully.

Checks

• I've added new tests (if relevant)
• I've added/updated the relevant documentation
• I've run the focused provider tests
• I've verified the provider against a live Sailbox environment

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[kiwigitops]

One behavior gap to check is write(..., user=...). The method runs _check_write_with_exec(..., user=user), but the actual sailbox.write(...) call that creates or replaces the file still runs through the provider file API without the requested user.

For a new file under a directory the user can write, the access check can pass, but the resulting file may be owned by the sandbox default/root user rather than the requested user. Other filesystem helpers treat user as the execution identity for the operation itself, so this could create surprising ownership/permission differences after a write. If Sailbox cannot write as a specific user directly, it may need a temp-file plus install/chown path through exec for the user case.

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