Validate JSON-RPC error data values

[leehack]

Summary

• validate JSON-RPC error data as a JSON value when parsing and serializing
• reject non-JSON Dart objects and non-finite numbers before transport encoding
• add edge-case coverage for valid nested data and invalid data values

Validation

• dart format .
• dart analyze
• git diff --check
• dart test
• cd packages/mcp_dart_cli && dart test test/src/conformance_command_test.dart
• cd packages/mcp_dart_cli && dart run mcp_dart_cli:mcp_dart conformance --suite all --json

Draft stacked PR for the MCP 2026-07-28 RC work. Base is the prior stack branch, not main.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.43%. Comparing base (2b5e414) to head (f5d80ae).

Additional details and impacted files

@@                        Coverage Diff                        @@
##           spec/elicitation-content-numbers     #237   +/-   ##
=================================================================
  Coverage                             88.43%   88.43%           
=================================================================
  Files                                    47       47           
  Lines                                  9313     9314    +1     
=================================================================
+ Hits                                   8236     8237    +1     
  Misses                                 1077     1077           

Flag	Coverage Δ
unittests	88.43% <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 #270 (JSON-RPC metadata hardening); the consolidated tip is tree-identical to the previous micro-stack tip.