Reject stateless server requests on response streams

[leehack]

Summary

• track 2026 stateless request response streams separately from legacy Streamable HTTP streams
• reject server-initiated JSON-RPC requests on stateless response streams so client input uses MRTR input-required results
• add a transport regression test and changelog entry

Validation

• dart format .
• dart analyze
• dart test test/server/streamable_https_test.dart
• dart test
• dart pub global run coverage:test_with_coverage
• git diff --check

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.57%. Comparing base (b886e8d) to head (edb7d6a).

Additional details and impacted files

@@                        Coverage Diff                        @@
##           spec/2026-stateless-post-only     #194      +/-   ##
=================================================================
+ Coverage                          86.55%   86.57%   +0.01%     
=================================================================
  Files                                 46       46              
  Lines                               8570     8578       +8     
=================================================================
+ Hits                                7418     7426       +8     
  Misses                              1152     1152              

Flag	Coverage Δ
unittests	86.57% <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 #268 (MCP 2026 transport enforcement); the consolidated tip is tree-identical to the previous micro-stack tip.