Add client subscription listen support

[leehack]

Summary

• add a client-side subscriptions/listen handle that correlates stream notifications by subscription id
• validate subscription acknowledgments and notification filters before dispatching stream events
• preserve _meta on typed list-changed notifications so stdio clients can demultiplex subscription streams

Validation

• dart analyze
• dart test test/types/subscriptions_test.dart
• dart test test/mcp_2026_07_28_test.dart --name "client .*subscription|listenSubscriptions"
• dart test
• dart pub global run coverage:test_with_coverage
• dart run bin/mcp_dart.dart conformance --suite all --json
• git diff --check

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.94%. Comparing base (152fd25) to head (e867775).

Additional details and impacted files

@@                          Coverage Diff                          @@
##           spec/2026-discovery-version-retry     #207      +/-   ##
=====================================================================
+ Coverage                              87.65%   87.94%   +0.28%     
=====================================================================
  Files                                     47       47              
  Lines                                   8821     8957     +136     
=====================================================================
+ Hits                                    7732     7877     +145     
+ Misses                                  1089     1080       -9     

Flag	Coverage Δ
unittests	87.94% <100.00%> (+0.28%)	⬆️

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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 #268 (MCP 2026 transport enforcement); the consolidated tip is tree-identical to the previous micro-stack tip.