feat(anthropic): update v1.3 integration and documentation

[seanbrar]

Summary

Update the Anthropic integration for the upcoming 1.3 release, adopting the new beta Files API for uploads and updating the documentation across all capability matrices to include Anthropic.

Related issue

None

Test plan

• Updated test_providers.py to match the new client.beta.files.upload method signature.
• Ran a comprehensive verify_anthropic.py script locally that successfully validated file upload capability, structured output, cache defaults, conversation continuity, and fan-out workflows against claude-haiku-4-5.
• Verified that all repository checks and tests (just check) pass successfully.
• Architectural guarantee / Boundary coverage: Re-used existing boundary tests for the updated interface.

Notes

The scripts/verify_anthropic.py script was used for live API workflow verification and then cleanly removed prior to commit.

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• PR title follows conventional commits
• make check passes
• Tests cover the meaningful cases, not just the happy path
• Docs updated (if this changes public API or user-facing behavior)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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