Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Inline comments:
In `@mlir/lib/Compiler/CompilerPipeline.cpp`:
- Around line 150-154: The `runPipeline` setup in `QuantumCompilerPipeline`
should validate `config_.decomposeMultiControlledMinControls` before any stages
mutate the module, instead of relying on `qco::createDecomposeMultiControlled`
to reject bad values later. Add an upfront config check alongside the other
early validations in `runPipeline`, using the `enableDecomposeMultiControlled` /
`decomposeMultiControlledMinControls` fields, and fail fast before Stage 1 if
the value is below 2.

In `@mlir/lib/Dialect/QCO/Transforms/Decomposition/McxSynthesis.cpp`:
- Around line 461-462: Clarify the one-dirty-ancilla branch in
McxSynthesis::decompose so the condition matches the comment’s “22+ controls”
wording and the parity-sensitive logic is unambiguous. Update the check on n to
explicitly reflect the intended control-count threshold in the dirty-ancilla
selection path, and keep the branch condition aligned with the surrounding
synthesis helpers so future cleanup does not invert the rule by mistake.

In `@mlir/unittests/Compiler/test_compiler_pipeline.cpp`:
- Around line 259-265: The CompilerPipelineTest end-to-end coverage only
exercises the X-path in DecomposeMultiControlledPass, so add a second
integration case for the Z-path using mlir::qc::multipleControlledZ. Update the
test fixture in test_compiler_pipeline.cpp by following the same pattern as the
existing runPipeline call and assert the MCZ pipeline behavior through the same
QCProgramBuilder::build and CompilationRecord flow. Keep the new case alongside
the current DecomposeMultiControlledPass coverage so both multipleControlledX
and multipleControlledZ are validated through the QC→QCO→optimization pipeline.

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Inline comments:
In `@mlir/unittests/Compiler/test_compiler_pipeline.cpp`:
- Around line 283-297: The test currently only checks that
QuantumCompilerPipeline::runPipeline fails, which still allows the rejection to
happen later in DecomposeMultiControlled after earlier stages have run. Update
CompilerPipelineTest.RejectsDecomposeMultiControlledMinControlsBelowTwo to also
assert that CompilationRecord::afterQCImport remains empty when
decomposeMultiControlledMinControls is below two, so the preflight validation is
enforced before Stage 1 or any intermediate records are produced.

In
`@mlir/unittests/Dialect/QCO/Transforms/Decomposition/test_multi_controlled_decomposition.cpp`:
- Around line 378-404: The two “untouched” tests only verify that one
multi-controlled op remains, so they can miss regressions where the surviving
operation changes kind; update McxDecompositionTest::LeavesMultiOpCtrlUntouched
and McxDecompositionTest::LeavesMultiControlledHUntouched to assert the
preserved op is still the expected CtrlOp/MCH form and that its body is
unchanged after runDecomposePass, using the existing QCOProgramBuilder setup and
countMultiControlledOps as a secondary check only.

---

Outside diff comments:
In `@mlir/unittests/Compiler/test_compiler_pipeline.cpp`:
- Around line 259-281: The decomposition tests in CompilerPipelineTest are
currently too permissive because runPipeline still enables an unrelated Stage 5
pass, so a change in MergeSingleQubitRotationGates could make the afterQCOCanon
versus afterOptimization comparison pass even if multi-controlled decomposition
fails. Update DecomposeMultiControlledPass and DecomposeMultiControlledPassMcz
to run with only the decomposition-relevant pipeline enabled, or otherwise
disable the Stage 5 rotation-merge path in runPipeline for these checks, so the
EXPECT_NE assertion specifically validates DecomposeMultiControlled and the
multipleControlledX/multipleControlledZ cases.