Harden Streamable HTTP response headers

[leehack]

Summary

• add the draft-recommended X-Accel-Buffering: no header to Streamable HTTP SSE responses
• centralize SSE response headers across GET, replay, and stateless raw socket responses
• mark JSON-RPC error response bodies as application/json and cover the header behavior in transport tests

Spec reference: https://modelcontextprotocol.io/specification/draft/basic/transports

Validation

• dart format .
• dart analyze
• dart test test/server/streamable_https_test.dart --name "X-Accel|response headers|stateless non-POST|routes a server-originated|GET Last-Event-ID|rejects server requests on response streams|validates required protocol header"
• 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 87.39%. Comparing base (e5843fe) to head (3ad4e83).

Additional details and impacted files

@@                             Coverage Diff                             @@
##           spec/2026-client-result-type-validation     #201      +/-   ##
===========================================================================
+ Coverage                                    86.79%   87.39%   +0.59%     
===========================================================================
  Files                                           46       46              
  Lines                                         8749     8686      -63     
===========================================================================
- Hits                                          7594     7591       -3     
+ Misses                                        1155     1095      -60     

Flag	Coverage Δ
unittests	87.39% <100.00%> (+0.59%)	⬆️

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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.