Add MCP 2026 stateless HTTP headers

[leehack]

Summary

• add stateless Streamable HTTP header validation for MCP-Protocol-Version, Mcp-Method, Mcp-Name, and present Mcp-Param-* headers
• make the Streamable HTTP client mirror 2026 request metadata into standard HTTP headers
• allow StreamableMcpServer to route 2026 stateless POST requests without an Mcp-Session-Id, while preserving legacy unknown-session behavior
• keep GET/DELETE on the 2026 stateless path as 405 Method Not Allowed through the transport

Stack

Base: #178 (spec/2026-07-28-rc)

This is the second PR in the MCP 2026-07-28 RC stack for #177.

Notes

This PR focuses on the standard Streamable HTTP stateless/header behavior. Full tool-schema-driven x-mcp-header discovery/filtering for client-emitted Mcp-Param-* headers can land as a separate follow-up if we want that isolated for review.

Validation

• dart analyze
• dart test test/server/streamable_https_test.dart test/client/streamable_https_test.dart test/server/streamable_mcp_server_test.dart test/mcp_2026_07_28_test.dart
• dart test -j 1

[codecov]

Codecov Report

❌ Patch coverage is 89.84772% with 20 lines in your changes missing coverage. Please review.
✅ Project coverage is 85.32%. Comparing base (4a81430) to head (b1bad7e).

Files with missing lines	Patch %	Lines
lib/src/server/streamable_https.dart	85.45%	16 Missing ⚠️
lib/src/server/streamable_mcp_server.dart	91.83%	4 Missing ⚠️

Additional details and impacted files

@@                  Coverage Diff                   @@
##           spec/2026-07-28-rc     #179      +/-   ##
======================================================
+ Coverage               85.20%   85.32%   +0.11%     
======================================================
  Files                      45       45              
  Lines                    7579     7770     +191     
======================================================
+ Hits                     6458     6630     +172     
- Misses                   1121     1140      +19     

Flag	Coverage Δ
unittests	85.32% <89.84%> (+0.11%)	⬆️

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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 #267 (MCP 2026 feature foundation); the consolidated tip is tree-identical to the previous micro-stack tip.