Validate result metadata JSON objects

[leehack]

Summary

• validate remaining typed result, descriptor, notification _meta fields with shared JSON-object validation
• validate generic JSON-RPC envelope params, result, and _meta maps against JSON values
• add stable 2025-11-25 and 2026-07-28 RC regression coverage

Validation

• dart format .
• dart analyze
• dart test test/types_test.dart test/mcp_2025_11_25_test.dart test/mcp_2026_07_28_test.dart
• dart test
• git diff --check
• (cd packages/mcp_dart_cli && dart run mcp_dart_cli:mcp_dart conformance --suite all --json)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.27%. Comparing base (529d22e) to head (70bf769).

Additional details and impacted files

@@                              Coverage Diff                              @@
##           spec/shared-json-object-helper-validation     #224      +/-   ##
=============================================================================
+ Coverage                                      88.22%   88.27%   +0.04%     
=============================================================================
  Files                                             47       47              
  Lines                                           9201     9233      +32     
=============================================================================
+ Hits                                            8118     8150      +32     
  Misses                                          1083     1083              

Flag	Coverage Δ
unittests	88.27% <100.00%> (+0.04%)	⬆️

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[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.