Add MCP 2026 task HTTP routing headers

[leehack]

Summary

• map tasks/get, tasks/update, and tasks/cancel to Mcp-Name from params.taskId for 2026 stateless HTTP requests
• enforce the same required task id name header on the Streamable HTTP server
• add client/server regression tests for task HTTP metadata routing

Validation

• dart analyze
• dart test test/client/streamable_https_test.dart test/server/streamable_https_test.dart test/types/tasks_extension_test.dart
• dart test -j 1
• git diff --check

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.75%. Comparing base (a9df586) to head (61600d5).

Additional details and impacted files

@@                        Coverage Diff                        @@
##           spec/2026-task-extension-wire     #184      +/-   ##
=================================================================
+ Coverage                          85.74%   85.75%   +0.01%     
=================================================================
  Files                                 46       46              
  Lines                               8234     8242       +8     
=================================================================
+ Hits                                7060     7068       +8     
  Misses                              1174     1174              

Flag	Coverage Δ
unittests	85.75% <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 #267 (MCP 2026 feature foundation); the consolidated tip is tree-identical to the previous micro-stack tip.