Validate content JSON object fields

[leehack]

Summary

• validate common content/resource _meta fields with shared JSON-object checks
• validate ResourceContents passthrough fields and resource_link.annotations during parsing and serialization
• add 2025-11-25 and 2026-07-28 RC regression coverage for non-JSON content object values

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)

Spec status: this is part of the RC spec work and is intentionally draft/stacked until the official spec release.

[codecov]

Codecov Report

❌ Patch coverage is 95.12195% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.07%. Comparing base (802ade1) to head (43b3f9f).

Files with missing lines	Patch %	Lines
lib/src/types/content.dart	95.12%	2 Missing ⚠️

Additional details and impacted files

@@                           Coverage Diff                            @@
##           spec/sampling-json-object-validation     #222      +/-   ##
========================================================================
+ Coverage                                 88.04%   88.07%   +0.02%     
========================================================================
  Files                                        47       47              
  Lines                                      9204     9217      +13     
========================================================================
+ Hits                                       8104     8118      +14     
+ Misses                                     1100     1099       -1     

Flag	Coverage Δ
unittests	88.07% <95.12%> (+0.02%)	⬆️

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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.