Reject stateless legacy task status notifications

[leehack]

Summary

• reject legacy notifications/tasks/status on MCP 2026 stateless server notification handling
• reject the same legacy notification before stateless clients send it
• cover both server and client regressions in the 2026 RC test suite

Spec context

• MCP 2026 RC moves tasks to the io.modelcontextprotocol/tasks extension
• SEP-2663 defines extension task notifications as notifications/tasks; the 2025-11-25 notifications/tasks/status shape is legacy and not wire-compatible

Validation

• dart format .
• git diff --check
• dart test test/mcp_2026_07_28_test.dart
• dart test test/types/tasks_extension_test.dart test/types/subscriptions_test.dart
• dart analyze
• dart test

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.48%. Comparing base (bdcca67) to head (c22ba3f).

Additional details and impacted files

@@                         Coverage Diff                         @@
##           spec/stateless-require-nested-meta     #248   +/-   ##
===================================================================
  Coverage                               88.48%   88.48%           
===================================================================
  Files                                      47       47           
  Lines                                    9345     9345           
===================================================================
  Hits                                     8269     8269           
  Misses                                   1076     1076           

Flag	Coverage Δ
unittests	88.48% <ø> (ø)

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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 #271 (type/schema validation); the consolidated tip is tree-identical to the previous micro-stack tip.