Enforce MCP 2026 task extension capabilities

[leehack]

Summary

• enforce task extension capability checks for stateless tasks/get, tasks/update, tasks/cancel, and task subscriptions
• reject removed legacy tasks/list and tasks/result methods under stateless MCP 2026 metadata
• keep legacy task execution metadata and task request hints out of stateless tool flows

Spec context

• SEP-2663 Tasks Extension: https://modelcontextprotocol.io/seps/2663-tasks-extension
• Tasks extension overview: https://modelcontextprotocol.io/extensions/tasks/overview
• MCP 2025-11-25 tasks: https://modelcontextprotocol.io/specification/2025-11-25/basic/utilities/tasks

Validation

• dart format .
• dart analyze
• dart test test/mcp_2026_07_28_test.dart
• dart test test/mcp_2025_11_25_test.dart test/server/tasks_test.dart test/client/task_client_test.dart test/types/tasks_extension_test.dart
• dart test -j 1
• git diff --check

[codecov]

Codecov Report

❌ Patch coverage is 92.40506% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 86.00%. Comparing base (61600d5) to head (c9a341c).

Files with missing lines	Patch %	Lines
lib/src/server/mcp_server.dart	86.36%	3 Missing ⚠️
lib/src/server/server.dart	94.73%	3 Missing ⚠️

Additional details and impacted files

@@                       Coverage Diff                       @@
##           spec/2026-task-http-routing     #185      +/-   ##
===============================================================
+ Coverage                        85.75%   86.00%   +0.24%     
===============================================================
  Files                               46       46              
  Lines                             8242     8294      +52     
===============================================================
+ Hits                              7068     7133      +65     
+ Misses                            1174     1161      -13     

Flag	Coverage Δ
unittests	86.00% <92.40%> (+0.24%)	⬆️

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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.