Prefer nested request metadata

[leehack]

Summary

• prefer spec-defined params._meta over non-spec top-level _meta when parsing request and notification metadata
• keep top-level _meta as a compatibility fallback when nested metadata is absent
• add stable 2025-11-25 and draft 2026-07-28 regression coverage for metadata precedence

Spec notes

• The current stable schema (2025-11-25) defines request metadata under request params.
• The draft 2026-07-28 schema requires request metadata under params._meta, including protocol-version metadata for stateless requests.
• This PR prevents top-level extras from overriding the spec-shaped nested metadata while preserving fallback parsing for older/non-conforming peers.

Sources:

• https://github.com/modelcontextprotocol/modelcontextprotocol/releases
• https://raw.githubusercontent.com/modelcontextprotocol/modelcontextprotocol/main/schema/2025-11-25/schema.json
• https://raw.githubusercontent.com/modelcontextprotocol/modelcontextprotocol/main/schema/draft/schema.json

Verification

• dart format lib/src/types/json_rpc.dart test/mcp_2025_11_25_test.dart test/mcp_2026_07_28_test.dart
• dart test test/mcp_2025_11_25_test.dart test/mcp_2026_07_28_test.dart
• dart analyze
• dart test
• (cd packages/mcp_dart_cli && dart analyze && dart test)
• git diff --check

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.45%. Comparing base (f13ede6) to head (463167d).

Additional details and impacted files

@@                            Coverage Diff                            @@
##           spec/discover-request-meta-validation     #245      +/-   ##
=========================================================================
- Coverage                                  88.45%   88.45%   -0.01%     
=========================================================================
  Files                                         47       47              
  Lines                                       9338     9335       -3     
=========================================================================
- Hits                                        8260     8257       -3     
  Misses                                      1078     1078              

Flag	Coverage Δ
unittests	88.45% <100.00%> (-0.01%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[leehack]

Closing this draft micro PR because the RC spec work has been consolidated into a smaller reviewable stack. This change is preserved in #270 (JSON-RPC metadata hardening); the consolidated tip is tree-identical to the previous micro-stack tip.