Validate icon wire fields

[leehack]

Summary

• validate McpIcon.fromJson required and optional string fields with FormatException
• reject malformed sizes arrays and unknown theme enum values instead of silently dropping them
• add direct and nested regression coverage for icon wire parsing

Validation

• dart format lib/src/types/content.dart test/types_test.dart
• dart analyze
• dart test test/types_test.dart
• dart test test/mcp_2025_11_25_test.dart
• dart test test/mcp_2026_07_28_test.dart
• git diff --check
• dart test

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.55%. Comparing base (1e1892b) to head (38b2752).

Additional details and impacted files

@@                               Coverage Diff                                @@
##           spec/elicitation-primitive-schema-validation     #256      +/-   ##
================================================================================
+ Coverage                                         88.53%   88.55%   +0.01%     
================================================================================
  Files                                                47       47              
  Lines                                              9405     9421      +16     
================================================================================
+ Hits                                               8327     8343      +16     
  Misses                                             1078     1078              

Flag	Coverage Δ
unittests	88.55% <100.00%> (+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 #271 (type/schema validation); the consolidated tip is tree-identical to the previous micro-stack tip.