Validate elicitation number schema by protocol

[leehack]

Summary

• enforce stable 2025 integer-only form elicitation number-schema keywords by default
• allow draft 2026 fractional number-schema keywords through protocol-aware parsing and MRTR input requests
• add stable/draft regression coverage and changelog entry

Tests

• dart test test/elicitation_test.dart test/mcp_2025_11_25_test.dart test/mcp_2026_07_28_test.dart
• dart analyze
• git diff --check
• dart test
• dart pub global run coverage:test_with_coverage

[codecov]

Codecov Report

❌ Patch coverage is 95.12195% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.62%. Comparing base (13d9fc9) to head (f33a4ca).

Files with missing lines	Patch %	Lines
lib/src/client/client.dart	77.77%	2 Missing ⚠️

Additional details and impacted files

@@                           Coverage Diff                            @@
##           spec/elicitation-content-integer-values     #261   +/-   ##
========================================================================
  Coverage                                    88.62%   88.62%           
========================================================================
  Files                                           47       47           
  Lines                                         9480     9498   +18     
========================================================================
+ Hits                                          8402     8418   +16     
- Misses                                        1078     1080    +2     

Flag	Coverage Δ
unittests	88.62% <95.12%> (+<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 #271 (type/schema validation); the consolidated tip is tree-identical to the previous micro-stack tip.