Add flowIn/flowOut Flow attribute pins to every node, displayed in the node header

[Copilot]

• Rename FlowAttribute → Flow in desc/attribute.py and attribute.py (+ backward-compat alias)
• Update all imports, isinstance checks, comments, and QML type strings to use "Flow"
• Move flow input/output attrs from _attributes to _internalAttributes in node.py
• Update node.py _applyExpr() to also iterate over _internalAttributes
• Fix attribute.py _applyExpr() to search _internalAttributes when looking for source attr
• Update QML Node.qml to use internalAttribute()/hasInternalAttribute() for flowIn/flowOut
• Update nodeFactory.py compatibility checks to account for flow attrs in internalInputs
• Remove if v is not None filter from outputs in toDict()
• Update tests (test_flowAttribute.py, test_compatibility.py, test_pipeline.py)
• All 257 backend tests pass, CodeQL: 0 alerts

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[codecov]

Codecov Report

❌ Patch coverage is 98.72340% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 83.78%. Comparing base (63031e1) to head (9f2c140).
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
meshroom/core/nodeFactory.py	89.47%	2 Missing ⚠️
meshroom/core/attribute.py	93.75%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #3048      +/-   ##
===========================================
+ Coverage    83.40%   83.78%   +0.37%     
===========================================
  Files           73       74       +1     
  Lines         9870    10101     +231     
===========================================
+ Hits          8232     8463     +231     
  Misses        1638     1638              

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