Fix-40738: Fix ExteriorAlgebra multiplication to preserve symbolic coefficients

[Hapsa21]

Fixes #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

• The title is concise and informative.
• The description explains in detail what this PR is about.
• 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.

[fchapoton]

this looks too complicated, as coercion should happen automatically

you need to add a doctest showing success where there was failure

[Hapsa21]

I've added the doctest