sagemathgh-41206: Fix fragile doctest sorting in multi_polynomial_ideal
    
### Description
This PR fixes a fragile doctest failure reported in sagemath#41193.

**The Issue:**
The failing test in
`src/sage/rings/polynomial/multi_polynomial_ideal.py` used `sorted(...,
key=str)` on a list of dictionaries. In Python, the string
representation of a dictionary depends on the insertion order of its
keys. Since the upstream library `msolve` likely changed its internal
insertion order in newer versions, `key=str` produced a different sort
order than expected, causing the test to fail despite the mathematical
results being correct.

**The Fix:**
I updated the test command to replace the unstable sort key with a
robust one. This sorts the dictionary items before stringifying,
ensuring the sorting relies on the dictionary *contents* rather than
their internal memory layout.

```python
# Old (fragile)
key=str

# New (robust)
key=lambda d: str(sorted(d.items()))
```
**Verification**
I verified that this change makes the sorting immune to dictionary
insertion order.

**Proof of Fix**: Running the following script demonstrates why
`key=str` failed and why the new key succeeds:
```python

dict_A = {'x': 100, 'y': 500}
dict_B = {'y': 500, 'x': 100}

#  ---Old Behavior (key=str)---
# str(dict_A) is "{'x': 100, 'y': 500}"
# str(dict_B) is "{'y': 500, 'x': 100}"
print(f"Old method equal? {str(dict_A) == str(dict_B)}")
# Result: False (This caused the test failure)

# --- New Behavior (The Fix) ---
my_key = lambda d: str(sorted(d.items()))
print(f"New method equal? {my_key(dict_A) == my_key(dict_B)}")
# Result: True (The sort is now stable)
```
This ensures the doctest passes reliably regardless of the upstream
library's behavior.

### **Issue**
Fixes sagemath#41193 Relates to sagemath#41192

### 📝 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.
- [x] I have created tests covering the changes.
- [ ] I have updated the documentation and checked the documentation
preview.

### ⌛ Dependencies

None.
    
URL: sagemath#41206
Reported by: Hetarth Jodha
Reviewer(s):

Author: Release Manager

src/sage/rings/polynomial/multi_polynomial_ideal.py

@@ -2617,7 +2617,7 @@ def variety(self, ring=None, *, algorithm='triangular_decomposition', proof=True
             sage: I = Ideal([x^2 - 1, y^2 - 1])                                         # needs sage.rings.finite_rings
             sage: sorted(I.variety(algorithm='msolve',          # optional - msolve, needs sage.rings.finite_rings
             ....:                  proof=False),
-            ....:        key=str)
+            ....:        key=lambda d: str(sorted(d.items()))
             [{y: 1, x: 1},
              {y: 1, x: 536870908},
              {y: 536870908, x: 1},