Fix macOS clang release segfault in QCO constant propagation CtrlOp handling

[Copilot]

Description

The macOS-26-intel clang release CI job failed with a SEGFAULT in QCOConstantPropagationTest.testUnsatisfiableHybridCombination.
The crash was caused by mutating the control vector while iterating it in handleCtrlOp, which is undefined behavior and surfaced on clang/macOS.

• Root cause
  • inCtrlValues was modified (std::erase) during iteration over the same container.
• Change
  • Iterate over a snapshot (ctrlCandidates) and keep mutating inCtrlValues safely.
  • Scope is limited to mlir/lib/Dialect/QCO/Transforms/Optimizations/ConstantPropagation.cpp.
• Code example

  const auto ctrlCandidates = inCtrlValues;
  for (const auto qCtrl : ctrlCandidates) {
    // existing logic; may erase from inCtrlValues
  }

Checklist

• The pull request only contains commits that are focused and relevant to this change.
• I have added appropriate tests that cover the new/changed functionality.
• I have updated the documentation to reflect these changes.
• I have added entries to the changelog for any noteworthy additions, changes, fixes, or removals.
• I have added migration instructions to the upgrade guide (if needed).
• The changes follow the project's style guidelines and introduce no new warnings.
• The changes are fully tested and pass the CI checks.
• I have reviewed my own code changes.

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[LiRem101]

I thought the Copilot analysis was on my fork and not here. Sorry for the unecessary pR.