Enable citations on provided ContentDocument (Anthropic)

[sjawhar]

Summary

Inspect parses Anthropic document citations from responses, but callers could not enable citations on ContentDocument inputs.

• Add the public ContentDocument.citations opt-in, defaulting to False.
• Send Anthropic document configuration citations={"enabled": true} only when enabled.
• Preserve backward compatibility and leave other providers unchanged.

Testing

• Add direct Anthropic document-block conversion tests for opted-in and default behavior.
• uv run make check
• uv run make test

[sjawhar]

Resubmission of #4480, rebased onto current main (that PR was auto-closed by a head-branch deletion during fork cleanup, without review).