Validate sampling JSON object fields

[leehack]

Summary

• add shared JSON-object validation that rejects non-string keys, non-JSON values, and non-finite nested numbers
• validate sampling object fields during parse and serialization, including tool-use input, sampling message metadata, content annotations, content _meta, and create-message metadata
• cover both MCP 2025-11-25 and MCP 2026-07-28 RC regression tests

Validation

• dart format .
• dart test test/types/sampling_test.dart
• dart test test/mcp_2025_11_25_test.dart
• dart test test/mcp_2026_07_28_test.dart
• dart analyze
• dart test
• git diff --check
• (cd packages/mcp_dart_cli && dart run mcp_dart_cli:mcp_dart conformance --suite all --json)

[codecov]

Codecov Report

❌ Patch coverage is 98.21429% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 88.21%. Comparing base (b1134cd) to head (4b37a45).

Files with missing lines	Patch %	Lines
lib/src/types/sampling.dart	98.00%	1 Missing ⚠️

Additional details and impacted files

@@                          Coverage Diff                           @@
##           spec/finite-json-number-validation     #221      +/-   ##
======================================================================
+ Coverage                               88.13%   88.21%   +0.07%     
======================================================================
  Files                                      47       47              
  Lines                                    9179     9204      +25     
======================================================================
+ Hits                                     8090     8119      +29     
+ Misses                                   1089     1085       -4     

Flag	Coverage Δ
unittests	88.21% <98.21%> (+0.07%)	⬆️

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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 #270 (JSON-RPC metadata hardening); the consolidated tip is tree-identical to the previous micro-stack tip.