docs: clarify structured output validation timing

[lavish0000]

Closes #1702

Summary:

• document that structured-output validation happens after the tool function returns
• clarify that validation failures do not roll back side effects and should be treated as implementation/schema mismatches
• add guidance for side-effectful tools, including operation-id and split-phase patterns

Testing:

• python3 -m uv run pytest tests/client/test_output_schema_validation.py

[maxisbey]

Thanks for the PR! Per our CONTRIBUTING.md, PRs require a corresponding issue with the ready for work label so we can agree on scope before reviewing code. Closing for now — please open an issue first and re-submit once it's triaged.

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