Add unit tests for TFQPauliSumCollector.collect (#1015)

This change implements unit tests for the `collect` method in
`TFQPauliSumCollector`, addressing a testing gap in
`batch_util_test.py`. The new tests cover:
- Standard Pauli Z observable (expected energy -1.0)
- Identity observable (expected energy 1.0)
- Mixed observable (Z + 2.0*I) (expected energy 1.0)

---------

Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>

Author: mhucka

tensorflow_quantum/core/ops/batch_util_test.py

@@ -308,6 +308,28 @@ def test_no_circuit(self, sim):
         self.assertDTypeEqual(results, np.int8)
         self.assertEqual(np.zeros(shape=(0, 0, 0)).shape, results.shape)
 
+    def test_pauli_sum_collector_collect(self):
+        """Test the collect method of TFQPauliSumCollector."""
+        qubit = cirq.GridQubit(0, 0)
+        circuit = cirq.Circuit(cirq.X(qubit))
+        samples_per_term = 100
+        sampler = cirq.Simulator()
+
+        test_cases = [
+            ("Pauli observable (Z)", cirq.PauliSum.wrap(cirq.Z(qubit)), -1.0),
+            ("Identity observable", cirq.PauliSum.wrap(cirq.I(qubit)), 1.0),
+            ("Mixed observable (Z + 2.0*I)",
+             cirq.Z(qubit) + 2.0 * cirq.I(qubit), 1.0),
+        ]
+
+        for name, observable, expected_energy in test_cases:
+            with self.subTest(name):
+                collector = batch_util.TFQPauliSumCollector(
+                    circuit, observable, samples_per_term=samples_per_term)
+                collector.collect(sampler)
+                self.assertAlmostEqual(collector.estimated_energy(),
+                                       expected_energy)
+
 
 if __name__ == '__main__':
     tf.test.main()