Fix operator precedence bug in exp_pauli qubit concatenation

In `PyKernel.exp_pauli()`, the `+` operator binds tighter than the
ternary `if/else`, causing `qubitsList` to be silently dropped when
`quantumVal` is not None. Add parentheses to ensure individual qubits
are always concatenated with the register.

Author: huaweil-nv

python/cudaq/kernel/kernel_builder.py

@@ -956,7 +956,7 @@ def exp_pauli(self, theta, *args):
             if len(qubitsList) > 0:
                 quantumVal = quake.ConcatOp(
                     quake.VeqType.get(),
-                    [quantumVal] if quantumVal is not None else [] +
+                    ([quantumVal] if quantumVal is not None else []) +
                     qubitsList).result
             quake.ExpPauliOp([], [thetaVal], [], [quantumVal],
                              pauli=pauliWordVal)

python/tests/builder/test_kernel_builder.py

@@ -1083,6 +1083,49 @@ def test_exp_pauli():
         kernel.exp_pauli(theta, qubits, invalidOp)
 
 
+def test_exp_pauli_register_and_qubits():
+    """Test that exp_pauli correctly concatenates a register with
+    individual qubits (regression test for operator precedence fix)."""
+    cudaq.reset_target()
+
+    # Case 1: register + individual qubit
+    kernel = cudaq.make_kernel()
+    qreg = kernel.qalloc(2)
+    q_extra = kernel.qalloc()
+    kernel.exp_pauli(1.0, qreg, q_extra, 'XXX')
+    ir = str(kernel)
+    assert 'quake.concat' in ir, \
+        "exp_pauli should concat register and individual qubits"
+
+    # Case 2: register + multiple individual qubits
+    kernel = cudaq.make_kernel()
+    qreg = kernel.qalloc(2)
+    q0 = kernel.qalloc()
+    q1 = kernel.qalloc()
+    kernel.exp_pauli(0.5, qreg, q0, q1, 'XXYY')
+    ir = str(kernel)
+    assert 'quake.concat' in ir, \
+        "exp_pauli should concat register and multiple individual qubits"
+
+    # Case 3: individual qubits only (no register) should still work
+    kernel = cudaq.make_kernel()
+    q0 = kernel.qalloc()
+    q1 = kernel.qalloc()
+    kernel.exp_pauli(1.0, q0, q1, 'XX')
+    ir = str(kernel)
+    assert 'quake.concat' in ir
+
+    # Case 4: register only (no individual qubits) should still work
+    kernel = cudaq.make_kernel()
+    qreg = kernel.qalloc(2)
+    kernel.exp_pauli(1.0, qreg, 'XX')
+    counts = cudaq.sample(kernel)
+    assert '00' in counts
+    assert '11' in counts
+    assert not '01' in counts
+    assert not '10' in counts
+
+
 def test_givens_rotation_op():
     cudaq.reset_target()
     angle = 0.2