Fix graphview signals

[TeunHuijben]

Fix GraphView signal emission to keep root and view consistent

Listeners attached to either root or view used to see one graph updated
while the other was still stale, because remove_node and update_node_attrs
emitted root's signal mid-mutation (before the view's local copy caught up).
add_node already did this correctly with a block-and-replay pattern.

• remove_node and update_node_attrs now block root's signal during the root
  call and re-emit at the end, mirroring add_node.
• Cache is_signal_on() results into locals so a slot connecting mid-call
  can't reference an unbound old_attrs.
• New regression tests in test_graph_view_signals.py assert that listeners
  on either side see consistent has_node / attribute values at signal time.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.73%. Comparing base (d657351) to head (4e5b7c7).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #287      +/-   ##
==========================================
+ Coverage   87.70%   87.73%   +0.02%     
==========================================
  Files          57       57              
  Lines        4879     4890      +11     
  Branches      858      862       +4     
==========================================
+ Hits         4279     4290      +11     
  Misses        378      378              
  Partials      222      222              

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