Add test for unused initializer check with graph outputs

[Copilot]

The fix prevents removing initializers that are graph outputs during constant folding. This test validates that behavior.

Changes

• Added test_initializer_as_graph_output_is_not_removed to verify initializers used in foldable nodes are preserved when they are graph outputs

Test Case

# Model with constant used in operation AND as graph output
agraph (float[N] x) => (float[N] y, float z) {
    constant = Constant <value_float=2.0> ()
    y = Mul(x, constant)
    z = Identity(constant)  # constant is also output
}

After constant folding:

• Identity node is removed
• Constant becomes initializer 'z'
• Initializer 'z' is NOT removed despite having no uses (it's a graph output)

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[justinchuby]

@gramalingam

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 70.08%. Comparing base (f958061) to head (b719f4a).
⚠️ Report is 4 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2733   +/-   ##
=======================================
  Coverage   70.07%   70.08%           
=======================================
  Files         226      226           
  Lines       27276    27285    +9     
  Branches     2754     2754           
=======================================
+ Hits        19113    19122    +9     
  Misses       7213     7213           
  Partials      950      950           

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