Add MCP 2026 task extension wire support

[leehack]

Summary

• add the MCP Tasks extension capability helper and task extension wire/result types
• parse tasks/update and notifications/tasks, including detailed task status payloads
• add taskIds to subscriptions/listen filters and reject task subscriptions without the tasks extension capability

Validation

• dart analyze
• dart test test/types/tasks_extension_test.dart test/types/subscriptions_test.dart test/mcp_2026_07_28_test.dart test/types_test.dart test/server/server_test.dart test/client/client_test.dart
• dart test -j 1
• git diff --check

[codecov]

Codecov Report

❌ Patch coverage is 86.18421% with 21 lines in your changes missing coverage. Please review.
✅ Project coverage is 85.74%. Comparing base (10c4973) to head (a9df586).

Files with missing lines	Patch %	Lines
lib/src/server/server.dart	67.74%	10 Missing ⚠️
lib/src/types/tasks.dart	91.83%	8 Missing ⚠️
lib/src/client/client.dart	62.50%	3 Missing ⚠️

Additional details and impacted files

@@                       Coverage Diff                        @@
##           spec/2026-subscriptions-listen     #183    +/-   ##
================================================================
  Coverage                           85.74%   85.74%            
================================================================
  Files                                  46       46            
  Lines                                8086     8234   +148     
================================================================
+ Hits                                 6933     7060   +127     
- Misses                               1153     1174    +21     

Flag	Coverage Δ
unittests	85.74% <86.18%> (+<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.