Validate JSON-RPC params member wire values

[leehack]

Summary

• Reject JSON-RPC request and notification envelopes when params is explicitly null
• Pre-validate params as an object before dispatching to typed request/notification parsers
• Add edge-case coverage plus a CLI conformance fixture for null params

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 (21bce6f) to head (d166cc1).

Additional details and impacted files

@@                            Coverage Diff                            @@
##           spec/json-rpc-error-object-validation     #241      +/-   ##
=========================================================================
- Coverage                                  88.43%   88.43%   -0.01%     
=========================================================================
  Files                                         47       47              
  Lines                                       9324     9322       -2     
=========================================================================
- Hits                                        8246     8244       -2     
  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.