Reject JSON-RPC responses with result and error

[leehack]

Summary

• reject JSON-RPC response envelopes that include both result and error
• add unit and CLI conformance coverage for the malformed response case
• update the unreleased MCP 2026 RC changelog

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

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.43%. Comparing base (f5d80ae) to head (eb37424).

Additional details and impacted files

@@                          Coverage Diff                          @@
##           spec/json-rpc-error-data-json-values     #238   +/-   ##
=====================================================================
  Coverage                                 88.43%   88.43%           
=====================================================================
  Files                                        47       47           
  Lines                                      9314     9314           
=====================================================================
  Hits                                       8237     8237           
  Misses                                     1077     1077           

Flag	Coverage Δ
unittests	88.43% <100.00%> (ø)

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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.