sagemathgh-40741: Fix-40738: Fix ExteriorAlgebra multiplication to preserve symbolic coefficients
    
Fixes sagemath#40738

**Problem:**

When multiplying elements of an `ExteriorAlgebra` over the Symbolic Ring
(`SR`), symbolic coefficients were being dropped in certain cases, such
as in the product `(x*a)*(y*b)`.

**Solution:**

This PR corrects the multiplication logic in
`ExteriorAlgebraElement._mul_` by:
1.  Defining `R = self.base_ring()` at the start of the method.
2.  Explicitly coercing the coefficients to the base ring (`R(cl) *
R(cr)`) before they are multiplied.

This ensures that symbolic multiplication is handled correctly,
preserving all coefficients as expected.

---

### ### Checklist

- [x] The title is concise and informative.
- [x] The description explains in detail what this PR is about.
- [x] I have linked a relevant issue or discussion.
- [ ] I have created a new test for this change.
- [ ] I have updated the documentation and checked the documentation
preview.

### ### Hourglass

- [ ] List all open PRs that this PR logically depends on.
    
URL: sagemath#40741
Reported by: Hetarth Jodha
Reviewer(s): Chenxin Zhong, Hetarth Jodha

Author: Release Manager

src/sage/algebras/clifford_algebra_element.pyx

@@ -442,8 +442,17 @@ cdef class ExteriorAlgebraElement(CliffordAlgebraElement):
             4*a*b*c*d + 4*a*b*c + 4*a*b*d + 4*a*c*d + 4*b*c*d
              + 2*a*b + 2*a*c + 2*a*d + 2*b*c + 2*b*d + 2*c*d
              + 2*a + 2*b + 2*c + 2*d + 1
+
+            sage: from sage.all import ExteriorAlgebra, SR, var
+            sage: L.<a, b> = ExteriorAlgebra(SR)
+            sage: x, y = var('x y')
+            sage: x * (y * b)
+            x*y*b
+            sage: (x * a) * (y * b)
+            x*y*a*b
         """
         cdef Parent P = self._parent
+        cdef R = P.base_ring()
         zero = P._base.zero()
         cdef dict d
         cdef ExteriorAlgebraElement rhs = <ExteriorAlgebraElement> other
@@ -514,7 +523,7 @@ cdef class ExteriorAlgebraElement(CliffordAlgebraElement):
                     if tot_cross % 2:
                         cr = -cr
 
-                val = d.get(t, zero) + cl * cr
+                val = d.get(t, zero) + R(cl) * R(cr)
                 if not val:
                     del d[t]
                 else: