gh-39059: Make multivariate ideal reduce() work on integer
    
Fixes #38560.

### 📝 Checklist

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- [x] The title is concise and informative.
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preview.
    
URL: #39059
Reported by: user202729
Reviewer(s): Travis Scrimshaw, user202729

Author: Release Manager

src/sage/rings/polynomial/multi_polynomial_ideal.py

@@ -248,6 +248,7 @@
 from sage.rings.integer_ring import ZZ
 from sage.rings.noncommutative_ideals import Ideal_nc
 from sage.rings.qqbar_decorators import handle_AA_and_QQbar
+from sage.structure.element import parent
 from sage.structure.richcmp import (op_EQ, op_GE, op_GT, op_LE, op_LT, op_NE,
                                     rich_to_bool, richcmp_method)
 from sage.structure.sequence import Sequence
@@ -4965,6 +4966,23 @@ def reduce(self, f):
 
             Requires computation of a Groebner basis, which can be a
             very expensive operation.
+
+        TESTS:
+
+        Check for :issue:`38560`::
+
+            sage: I.reduce(1)
+            1
+            sage: I.reduce(1r)
+            1
+            sage: I.reduce(pi.n())
+            Traceback (most recent call last):
+            ...
+            TypeError: no canonical coercion from Real Field with 53 bits of precision to Multivariate Polynomial Ring in x, y over Rational Field
+            sage: I.reduce(float(pi.n()))
+            Traceback (most recent call last):
+            ...
+            TypeError: no canonical coercion from <class 'float'> to Multivariate Polynomial Ring in x, y over Rational Field
         """
         try:
             strat = self._groebner_strategy()
@@ -4973,6 +4991,7 @@ def reduce(self, f):
             pass
 
         gb = self.groebner_basis()
+        f = self.ring().coerce(f)
         return f.reduce(gb)
 
     def _contains_(self, f):