Reject non-integer elicitation content

[leehack]

Summary

• tighten ElicitResult.content validation to reject non-integer numeric values
• cover both parse and serialization paths with regression tests
• document the stable and MCP 2026 schema alignment in the changelog

Tests

• dart format .
• dart analyze
• dart test test/elicitation_test.dart test/mcp_2025_11_25_test.dart 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 88.03%. Comparing base (c955a2d) to head (b7543b0).

Additional details and impacted files

@@                         Coverage Diff                          @@
##           spec/2026-input-response-validation     #214   +/-   ##
====================================================================
  Coverage                                88.03%   88.03%           
====================================================================
  Files                                       47       47           
  Lines                                     9077     9077           
====================================================================
  Hits                                      7991     7991           
  Misses                                    1086     1086           

Flag	Coverage Δ
unittests	88.03% <100.00%> (ø)

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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.