Require enum for elicitation enum names

[leehack]

Summary

• reject form elicitation schemas that include legacy enumNames without enum
• cover both JSON parsing and typed serialization paths
• document the stable and MCP 2026 schema alignment in the changelog

Tests

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

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.03%. Comparing base (b7543b0) to head (2b82169).

Additional details and impacted files

@@                         Coverage Diff                         @@
##           spec/elicit-result-integer-content     #215   +/-   ##
===================================================================
  Coverage                               88.03%   88.03%           
===================================================================
  Files                                      47       47           
  Lines                                    9077     9078    +1     
===================================================================
+ Hits                                     7991     7992    +1     
  Misses                                   1086     1086           

Flag	Coverage Δ
unittests	88.03% <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 #269 (MCP 2026 result semantics); the consolidated tip is tree-identical to the previous micro-stack tip.