fix(rmcp): preserve raw responses through OAuth and header caching

[jmagar]

Summary

• Forward raw-response support through AuthClient so typed extension results such as native Skills survive OAuth-wrapped HTTP clients.
• Maintain tool parameter-header schema caching for raw tools/list responses while preserving extension fields and filtering invalid annotations.
• Add OAuth transport and raw schema-cache regression coverage.

Verification

• SDK custom-request tests: 7 passed.
• SDK HTTP transport unit tests: 8 passed.
• Labby integration with this revision via local override: 6 native Skills E2E cases passed; full gateway suite 1213 passed with four workers, five skipped.
• Labby strict integration-target Clippy passed.
• SDK strict Clippy reports 12 diagnostics on unchanged code; not claimed green.

No merge or deployment is included.

[jmagar]

Review follow-up: code, errors, types, tests, comments/docs, and simplification passes completed. Added the requested worker-level JSON tools/list -> tools/call header regression plus legacy/malformed raw-response cases; custom tests 8/8 and transport unit tests 9/9 pass. Final affected-pass review found no actionable issues. CI exposed a stale large_enum_variant expectation; changed it to a targeted allow for the documented allocation tradeoff. The latest CI Clippy check passes. Labby now consumes published f94e8fa without local overrides and its native Skills E2E suite passes 6/6. No merge or deployment performed.