Validate JSON-RPC error object wire values

[leehack]

Summary

• Validate JSON-RPC error response error members as JSON objects before parsing
• Reject missing or invalid error code / message fields with FormatException instead of Dart cast errors
• Add edge-case coverage plus a CLI conformance fixture for malformed error objects

Validation

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

Draft stacked PR for RC spec work. Do not merge/release to main until the spec is finalized.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.43%. Comparing base (e7cdefd) to head (21bce6f).

Additional details and impacted files

@@                          Coverage Diff                          @@
##           spec/json-rpc-method-wire-validation     #240   +/-   ##
=====================================================================
  Coverage                                 88.43%   88.43%           
=====================================================================
  Files                                        47       47           
  Lines                                      9319     9324    +5     
=====================================================================
+ Hits                                       8241     8246    +5     
  Misses                                     1078     1078           

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.