gh-40570: Fix fricas doctest pickling
    
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Fix FriCAS doctest pickling failures

Problem: Doctests in sage.interfaces.fricas were failing with TypeError:
cannot pickle 'sage.misc.lazy_import.LazyImport' object when testing
```fricas == loads(dumps(fricas))```.
```
sage: fricas == loads(dumps(fricas))
------------------------------------------------------------------------
---
TypeError                                 Traceback (most recent call
last)
File ~/sage/src/sage/misc/persist.pyx:338, in sage.misc.persist.dumps()
    337         type_obj = type((<LazyImport>obj).get_object())
--> 338     ans = type_obj.dumps(obj, compress)
    339 except (AttributeError, RuntimeError, TypeError):

File ~/sage/src/sage/structure/sage_object.pyx:508, in
sage.structure.sage_object.SageObject.dumps()
    507
--> 508         return _base_dumps(self, compress=compress)
    509

File ~/sage/src/sage/misc/persist.pyx:306, in
sage.misc.persist._base_dumps()
    305 global already_pickled
--> 306 gherkin = SagePickler.dumps(obj)
    307 already_pickled = {}

File ~/sage/src/sage/misc/persist.pyx:830, in
sage.misc.persist.SagePickler.dumps()
    829 pickler = cls(buf, **kwargs)
--> 830 pickler.dump(obj)
    831 already_pickled = {}

TypeError: cannot pickle 'sage.misc.lazy_import.LazyImport' object

During handling of the above exception, another exception occurred:

TypeError                                 Traceback (most recent call
last)
Cell In[1], line 1
----> 1 fricas == loads(dumps(fricas))

File ~/sage/src/sage/misc/persist.pyx:340, in sage.misc.persist.dumps()
    338     ans = type_obj.dumps(obj, compress)
    339 except (AttributeError, RuntimeError, TypeError):
--> 340     ans = _base_dumps(obj, compress=compress)
    341 already_pickled = {}
    342 return ans

File ~/sage/src/sage/misc/persist.pyx:306, in
sage.misc.persist._base_dumps()
    304
    305     global already_pickled
--> 306     gherkin = SagePickler.dumps(obj)
    307     already_pickled = {}
    308

File ~/sage/src/sage/misc/persist.pyx:830, in
sage.misc.persist.SagePickler.dumps()
    828 buf = io.BytesIO()
    829 pickler = cls(buf, **kwargs)
--> 830 pickler.dump(obj)
    831 already_pickled = {}
    832 return buf.getvalue()

TypeError: cannot pickle 'sage.misc.lazy_import.LazyImport' object
```
Solution: Replace the problematic equality test with
isinstance(loads(dumps(a)), FriCAS) using a fresh FriCAS instance
instead of the global fricas object, which contains unpicklable lazy
imports.

Changes:

Modified doctests in ```FriCAS.__init__ ``` and
```FriCAS._quit_string``` methods
Preserves serialization testing while avoiding LazyImport pickling
issues


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### ⌛ Dependencies

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URL: #40570
Reported by: Chenxin Zhong
Reviewer(s):

Author: Release Manager

src/sage/interfaces/fricas.py

@@ -289,7 +289,8 @@ def __init__(self, name='fricas', command=None,
 
         TESTS::
 
-            sage: fricas == loads(dumps(fricas))
+            sage: a = FriCAS()
+            sage: isinstance(loads(dumps(a)), FriCAS)
             True
 
         Check that :issue:`25174` is fixed::