Validate JSON-RPC error response ids

[leehack]

Summary

• Reject JSON-RPC error responses when id is explicitly null
• Keep omitted error-response IDs valid for malformed-request cases where the request ID could not be read
• Add edge-case, stdio raw-input, and CLI conformance coverage

Validation

• dart format .
• dart analyze
• git diff --check
• dart test test/types_edge_cases_test.dart test/server/stdio_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 (d166cc1) to head (9e9e0fc).

Additional details and impacted files

@@                           Coverage Diff                           @@
##           spec/json-rpc-params-object-validation     #242   +/-   ##
=======================================================================
  Coverage                                   88.43%   88.43%           
=======================================================================
  Files                                          47       47           
  Lines                                        9322     9322           
=======================================================================
  Hits                                         8244     8244           
  Misses                                       1078     1078           

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.