Consolidate MCP 2026 transport enforcement

[leehack]

Summary

• consolidate stateless transport enforcement, HTTP response/header behavior, removed-RPC guards, result-type validation, discovery retry, and client subscription handling
• keeps the transport behavior reviewable as one layer above the MCP 2026 feature foundation
• replaces the transport/enforcement portion of the prior micro-PR stack (Add MCP 2026 cacheable result hints #186-Allow JSON-value structured tool output #208)

Validation

• dart format --output=none --set-exit-if-changed .
• dart analyze
• git diff --check
• dart test
• dart pub global run coverage:test_with_coverage

Stacked on #267. This is a draft RC PR and should stay off main until the official spec release.

[codecov]

Codecov Report

❌ Patch coverage is 98.28375% with 15 lines in your changes missing coverage. Please review.
✅ Project coverage is 87.94%. Comparing base (7348727) to head (f3542a8).
⚠️ Report is 5 commits behind head on dev/2026-07-28-rc.

Files with missing lines	Patch %	Lines
lib/src/server/streamable_https.dart	95.27%	11 Missing ⚠️
lib/src/server/server.dart	97.72%	2 Missing ⚠️
lib/src/server/mcp_server.dart	99.10%	1 Missing ⚠️
lib/src/types/json_rpc.dart	0.00%	1 Missing ⚠️

Additional details and impacted files

@@                  Coverage Diff                  @@
##           dev/2026-07-28-rc     #268      +/-   ##
=====================================================
+ Coverage              86.00%   87.94%   +1.94%     
=====================================================
  Files                     46       47       +1     
  Lines                   8294     8957     +663     
=====================================================
+ Hits                    7133     7877     +744     
+ Misses                  1161     1080      -81     

Flag	Coverage Δ
unittests	87.94% <98.28%> (+1.94%)	⬆️

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