refactor(anthropic): deduplicate sequencing, media-type, and test helpers

[dgageot]

Summary

Simplify and deduplicate code in the Anthropic provider without changing any behavior.

Changes

• Move hashFile to test file — It was only used in tests; removed from files.go, added to files_test.go.
• Remove marshalToMapBeta/contentArrayBeta aliases — These were trivial aliases (var marshalToMapBeta = marshalToMap). All call sites now use the originals directly.
• Unify sequencing validation/repair with generics — Extract validateSequencing[T] and repairSequencing[T] to replace the nearly identical standard and beta implementations, eliminating ~80 lines of duplicated logic.
• Replace duplicated media-type switch with extractMediaType — Both convertUserMultiContent and convertBetaUserMultiContent had identical inline switch blocks for parsing data: image URLs. Now they call the existing extractMediaType helper.

Result

-87 lines net across 6 files. No features changed, all tests pass, linter clean.

[docker-agent]

Review Summary

Assessment: 🟢 APPROVE

This refactoring PR successfully deduplicates code in the Anthropic provider without introducing any bugs:

✅ hashFile moved to test file — Correctly relocated to files_test.go since it's only used in tests
✅ Removed trivial aliases — marshalToMapBeta/contentArrayBeta were just aliases; tests now use originals directly
✅ Generic sequencing functions — validateSequencing[T] and repairSequencing[T] correctly unify standard and Beta API logic using marshal-to-map inspection
✅ extractMediaType helper — Properly eliminates duplicated media-type switch statements in both converters

The generic implementations maintain identical behavior to the original code while reducing duplication. The makeSynthetic callback correctly constructs type-appropriate synthetic messages for each API variant.

Result: -87 lines net with no behavioral changes. All changed code is correct.