Validate enum wire fields

[leehack]

Summary

• add shared enum wire-value parsing for protocol enum fields
• reject malformed logging levels and sampling enum fields with FormatException
• validate toolChoice.mode during request parsing while preserving legacy raw map storage

Validation

• dart format lib/src/types/validation.dart lib/src/types/logging.dart lib/src/types/sampling.dart test/types/logging_types_test.dart test/types/sampling_test.dart test/mcp_2025_11_25_test.dart
• dart test test/types/logging_types_test.dart test/types/sampling_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 #265. 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 88.81%. Comparing base (f891c10) to head (e12088a).

Additional details and impacted files

@@                      Coverage Diff                      @@
##           spec/role-wire-validation     #266      +/-   ##
=============================================================
+ Coverage                      88.79%   88.81%   +0.02%     
=============================================================
  Files                             47       47              
  Lines                           9621     9632      +11     
=============================================================
+ Hits                            8543     8555      +12     
+ Misses                          1078     1077       -1     

Flag	Coverage Δ
unittests	88.81% <100.00%> (+0.02%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[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.