Reject stateless server requests on clients

[leehack]

Summary

• reject incoming server-initiated JSON-RPC requests after a client negotiates stateless MCP 2026
• align generic transports with the existing Streamable HTTP stateless response-stream behavior
• add 2026 RC regression coverage for direct transport-delivered server requests

Validation

• dart format .
• dart analyze
• dart test test/mcp_2026_07_28_test.dart
• dart test test/client/client_test.dart test/mcp_2025_11_25_test.dart
• dart test
• git diff --check
• (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 88.29%. Comparing base (4a0c9df) to head (2453e71).

Additional details and impacted files

@@                            Coverage Diff                            @@
##           spec/stateless-client-removed-rpc-guards     #226   +/-   ##
=========================================================================
  Coverage                                     88.29%   88.29%           
=========================================================================
  Files                                            47       47           
  Lines                                          9249     9252    +3     
=========================================================================
+ Hits                                           8166     8169    +3     
  Misses                                         1083     1083           

Flag	Coverage Δ
unittests	88.29% <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.