fix Tensor with jitted projector (#5720)

**Context:**

We are getting legacy op math failures:


https://github.com/PennyLaneAI/pennylane/actions/runs/9167752473/job/25205404890

Due to PR #5595 . When `Projector` started letting tracers through,
`Tensor` was unable to handle them.

**Description of the Change:**

Updates `Tensor.matrix` to handle matrices that have an ML interface.

**Benefits:**

No more bugs.

**Possible Drawbacks:**

**Related GitHub Issues:**

doc/releases/changelog-dev.md

@@ -132,6 +132,9 @@
 
 <h3>Bug fixes 🐛</h3>
 
+* The legacy `Tensor` class can now handle a `Projector` with abstract tracer input.
+  [(#5720)](https://github.com/PennyLaneAI/pennylane/pull/5720)
+
 * Fixed a bug that raised an error regarding expected vs actual `dtype` when using `JAX-JIT` on a circuit that 
   returned samples of observables containing the `qml.Identity` operator.
   [(#5607)](https://github.com/PennyLaneAI/pennylane/pull/5607)

pennylane/operation.py

@@ -2409,7 +2409,7 @@ def matrix(self, wire_order=None):
             # append diagonalizing unitary for specific wire to U_list
             U_list.append(mats[0])
 
-        mat_size = np.prod([np.shape(mat)[0] for mat in U_list])
+        mat_size = np.prod([qml.math.shape(mat)[0] for mat in U_list])
         wire_size = 2 ** len(self.wires)
         if mat_size != wire_size:
             if partial_overlap:
@@ -2428,7 +2428,7 @@ def matrix(self, wire_order=None):
 
         # Return the Hermitian matrix representing the observable
         # over the defined wires.
-        return functools.reduce(np.kron, U_list)
+        return functools.reduce(qml.math.kron, U_list)
 
     def check_wires_partial_overlap(self):
         r"""Tests whether any two observables in the Tensor have partially

tests/test_operation.py

@@ -2061,6 +2061,20 @@ def test_matmul_not_implemented(self):
         with pytest.raises(TypeError, match="unsupported operand type"):
             _ = op @ 1.0
 
+    @pytest.mark.jax
+    def test_matrix_jax_projector(self):
+        """Test that matrix can be computed with a jax projector."""
+
+        import jax
+
+        def f(state):
+            op = qml.Projector(state, wires=0)
+            return qml.operation.Tensor(op, qml.Z(1)).matrix()
+
+        res = jax.jit(f)(jax.numpy.array([0, 1]))
+        expected = np.array([[0, 0, 0, 0], [0, 0, 0, 0], [0, 0, 1, 0], [0, 0, 0, -1]])
+        assert qml.math.allclose(res, expected)
+
 
 with qml.operation.disable_new_opmath_cm():
     equal_obs = [