Validate elicitation primitive schema keywords

[leehack]

Summary

• enforce primitive elicitation requestedSchema keyword value types from the stable and draft MCP schemas
• reject malformed string, number, boolean, array, and top-level schema keywords at the ElicitRequest boundary
• expand elicitation regression coverage for valid and invalid primitive schema variants

Validation

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

[codecov]

Codecov Report

❌ Patch coverage is 97.82609% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 88.53%. Comparing base (fbff7e0) to head (1e1892b).

Files with missing lines	Patch %	Lines
lib/src/types/elicitation.dart	97.82%	1 Missing ⚠️

Additional details and impacted files

@@                           Coverage Diff                            @@
##           spec/discover-result-type-validation     #255      +/-   ##
========================================================================
+ Coverage                                 88.49%   88.53%   +0.04%     
========================================================================
  Files                                        47       47              
  Lines                                      9363     9405      +42     
========================================================================
+ Hits                                       8286     8327      +41     
- Misses                                     1077     1078       +1     

Flag	Coverage Δ
unittests	88.53% <97.82%> (+0.04%)	⬆️

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.