Validate capability object entries

[leehack]

Summary

• reject non-object experimental capability values for client and server capabilities
• reject non-object extension capability values during wire parsing
• update existing capability tests to use empty objects for feature markers and cover malformed stateless metadata

Spec evidence

• MCP 2025-11-25 and MCP 2026-07-28 RC schemas define capabilities.experimental entries as JSON objects
• MCP 2026-07-28 RC schemas define capabilities.extensions entries as JSON objects

Validation

• dart format .
• dart analyze
• dart test test/types_test.dart test/types_edge_cases_test.dart test/client/client_test.dart test/integration/completions_capability_test.dart test/mcp_2026_07_28_test.dart test/mcp_2025_11_25_test.dart test/server/server_validation_test.dart test/server/server_test.dart
• dart test
• dart run mcp_dart_cli:mcp_dart conformance --suite all --json (from packages/mcp_dart_cli)

[codecov]

Codecov Report

❌ Patch coverage is 93.47826% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.09%. Comparing base (184c37e) to head (caa5a9a).

Files with missing lines	Patch %	Lines
lib/src/types/initialization.dart	93.47%	3 Missing ⚠️

Additional details and impacted files

@@                              Coverage Diff                              @@
##           spec/input-required-capability-validation     #218      +/-   ##
=============================================================================
+ Coverage                                      88.08%   88.09%   +0.01%     
=============================================================================
  Files                                             47       47              
  Lines                                           9121     9155      +34     
=============================================================================
+ Hits                                            8034     8065      +31     
- Misses                                          1087     1090       +3     

Flag	Coverage Δ
unittests	88.09% <93.47%> (+0.01%)	⬆️

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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 #269 (MCP 2026 result semantics); the consolidated tip is tree-identical to the previous micro-stack tip.