Enforce MCP 2026 stateless POST-only transport

[leehack]

Summary

• skip legacy Streamable HTTP GET SSE setup and DELETE session termination when the client transport is using an MCP 2026 stateless protocol version
• route every stateless non-POST server request through the POST-only rejection path with Allow: POST
• cover low-level and high-level Streamable HTTP behavior while preserving 2025 session semantics

Validation

• dart format .
• dart analyze
• dart test test/client/streamable_https_test.dart
• dart test test/server/streamable_https_test.dart
• dart test test/server/streamable_mcp_server_test.dart
• dart test
• dart pub global run coverage:test_with_coverage

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.55%. Comparing base (4bb3d95) to head (b886e8d).

Additional details and impacted files

@@                             Coverage Diff                             @@
##           spec/2026-x-mcp-header-number-rejection     #193      +/-   ##
===========================================================================
- Coverage                                    86.57%   86.55%   -0.02%     
===========================================================================
  Files                                           46       46              
  Lines                                         8563     8570       +7     
===========================================================================
+ Hits                                          7413     7418       +5     
- Misses                                        1150     1152       +2     

Flag	Coverage Δ
unittests	86.55% <100.00%> (-0.02%)	⬇️

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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 #268 (MCP 2026 transport enforcement); the consolidated tip is tree-identical to the previous micro-stack tip.