Accept numeric JSON-RPC ids and progress tokens

[leehack]

Summary

• accept finite numeric JSON-RPC request IDs and MCP progress tokens in addition to strings
• validate SDK serialization paths so non-finite numbers are rejected before reaching the wire
• expand edge-case tests and CLI conformance fixtures for numeric IDs/tokens

Spec Context

• Official MCP stable and draft schemas define RequestId and ProgressToken as string | number.
• This PR keeps the RC stack draft-only and targets spec/mrtr-input-response-client-result-shapes, not main.

Verification

• dart format .
• dart analyze
• git diff --check
• dart test
• dart test -j 1
• 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

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.40%. Comparing base (f1564e1) to head (8498c2a).

Additional details and impacted files

@@                                Coverage Diff                                @@
##           spec/mrtr-input-response-client-result-shapes     #232      +/-   ##
=================================================================================
+ Coverage                                          88.38%   88.40%   +0.01%     
=================================================================================
  Files                                                 47       47              
  Lines                                               9308     9312       +4     
=================================================================================
+ Hits                                                8227     8232       +5     
+ Misses                                              1081     1080       -1     

Flag	Coverage Δ
unittests	88.40% <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.