Cancel stateless SSE requests on client close

[leehack]

Summary

• Align 2026 stateless Streamable HTTP request-specific SSE response streams with the draft requirement that closing the response stream cancels the in-flight request.
• Track detached stateless response sockets so client closure is observable and pending request IDs are converted into notifications/cancelled.
• Keep stateless response streams out of EventStore replay and add regression coverage.

Spec Source

• https://modelcontextprotocol.io/specification/draft/basic/transports

Validation

• dart format .
• dart analyze
• dart test test/server/streamable_https_test.dart --name "2026 stateless HTTP cancels pending request when SSE response closes"
• dart test test/server/streamable_https_test.dart
• dart test
• dart pub global run coverage:test_with_coverage
• git diff --check

[codecov]

Codecov Report

❌ Patch coverage is 87.20930% with 11 lines in your changes missing coverage. Please review.
✅ Project coverage is 86.69%. Comparing base (596459f) to head (3812440).

Files with missing lines	Patch %	Lines
lib/src/server/streamable_https.dart	87.20%	11 Missing ⚠️

Additional details and impacted files

@@                         Coverage Diff                         @@
##           spec/2026-ignore-session-header     #199      +/-   ##
===================================================================
- Coverage                            86.70%   86.69%   -0.01%     
===================================================================
  Files                                   46       46              
  Lines                                 8656     8727      +71     
===================================================================
+ Hits                                  7505     7566      +61     
- Misses                                1151     1161      +10     

Flag	Coverage Δ
unittests	86.69% <87.20%> (-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 #268 (MCP 2026 transport enforcement); the consolidated tip is tree-identical to the previous micro-stack tip.