Validate role wire fields

[leehack]

Summary

• add shared validation for MCP Role wire values (user or assistant)
• use the role validator for prompt messages, sampling messages, and create-message results
• keep the typed enum APIs unchanged while rejecting malformed wire JSON as FormatException

Validation

• dart format lib/src/types/validation.dart lib/src/types/sampling.dart lib/src/types/prompts.dart test/types/sampling_test.dart test/types_test.dart test/mcp_2025_11_25_test.dart
• dart test test/types/sampling_test.dart test/types_test.dart test/mcp_2025_11_25_test.dart
• dart analyze
• git diff --check
• dart test
• dart pub global run coverage:test_with_coverage

Stacked on #264. This remains a draft RC PR and stays off main until the official spec release.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.79%. Comparing base (370996f) to head (f891c10).

Additional details and impacted files

@@                         Coverage Diff                          @@
##           spec/annotations-wire-validation     #265      +/-   ##
====================================================================
+ Coverage                             88.77%   88.79%   +0.01%     
====================================================================
  Files                                    47       47              
  Lines                                  9615     9621       +6     
====================================================================
+ Hits                                   8536     8543       +7     
+ Misses                                 1079     1078       -1     

Flag	Coverage Δ
unittests	88.79% <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.