Validate implementation wire fields

[leehack]

Summary

• validate required and optional Implementation string fields with FormatException
• reject malformed icons lists and present-null optional fields in wire JSON
• require websiteUrl to be an absolute URI on parse and serialization

Validation

• dart format lib/src/types/initialization.dart test/types_test.dart
• dart analyze
• dart test test/types_test.dart --name Implementation
• 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 88.59%. Comparing base (eff3be4) to head (846a480).

Additional details and impacted files

@@                     Coverage Diff                      @@
##           spec/icon-uri-validation     #258      +/-   ##
============================================================
+ Coverage                     88.57%   88.59%   +0.02%     
============================================================
  Files                            47       47              
  Lines                          9432     9456      +24     
============================================================
+ Hits                           8354     8378      +24     
  Misses                         1078     1078              

Flag	Coverage Δ
unittests	88.59% <100.00%> (+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 #271 (type/schema validation); the consolidated tip is tree-identical to the previous micro-stack tip.