Fix default port hover callbacks

[pavanpodila]

Summary

• wire default port hover changes through every node render layer
• dispatch port mouse-enter and mouse-leave events from the editor
• add a widget regression test covering a real mouse entering and leaving a default port

Verification

• flutter analyze packages/vyuh_node_flow
• flutter test packages/vyuh_node_flow/test/widget/node_flow_editor_test.dart
• flutter test packages/vyuh_node_flow (6,844 tests)

Fixes #35

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.79%. Comparing base (701663a) to head (dd047f9).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #38      +/-   ##
==========================================
+ Coverage   88.33%   88.79%   +0.46%     
==========================================
  Files         143      143              
  Lines        9713     9722       +9     
==========================================
+ Hits         8580     8633      +53     
+ Misses       1133     1089      -44     

Flag	Coverage Δ
unittests	88.79% <100.00%> (+0.46%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
...h_node_flow/lib/src/editor/layers/nodes_layer.dart	84.90% <100.00%> (+0.29%)	⬆️
...yuh_node_flow/lib/src/editor/node_flow_editor.dart	74.14% <100.00%> (+0.20%)	⬆️
...b/src/editor/node_flow_editor_widget_handlers.dart	68.00% <100.00%> (+8.00%)	⬆️

... and 3 files with indirect coverage changes

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