fix(anthropic): bypass model_dump on content_block_stop

[chaynabors]

Anthropic SDK 0.84.0+ types stream events as ParsedMessageStreamEvent, which makes content_block_stop carry a generic ParsedTextBlock that does not match the discriminated union, triggering six Pydantic serialization warnings per invocation. The fix mirrors the existing message_stop workaround from #1746: build the minimal {type, index} dict that format_chunk already consumes instead of round-tripping through model_dump.

Fixes #1865.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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[awsarron]

Please add test coverage

[mkmeral]

/strands review

[github-actions]

Assessment: Approve

Small, focused bugfix that correctly mirrors the established message_stop workaround (#1746) to suppress Pydantic serialization warnings on content_block_stop. I verified the fix and ran the tests locally (passing). One minor maintainability suggestion left inline.

Review Notes

• Correctness: The targeted bypass is right — content_block_stop is the event that carries the accumulated ParsedTextBlock failing the discriminated union, and format_chunk already consumes the minimal {type, index} dict. The other else-branch events don't carry the offending block, so this should fully resolve [BUG] Strands Agent AnthropicModel emits Pydantic serialization warnings with Anthropic SDK v0.84.0 #1865.
• Testing: Good regression guard — model_dump() is wired to emit the warning, so a regression to the else branch would fail the test. Coverage confirmed by Codecov.
• Maintainability: The new branch lacks the WHY comment its message_stop sibling has (inline note).

Nicely scoped fix that follows the existing precedent.