BUG: avoid float upcast when mixing signed/unsigned ints in isin

[divya1974]

…ts added)

• closes BUG: Implicit conversion to float64 with isin() #61676 61676
• Tests added and passed if fixing a bug or adding a new feature
• All code checks passed.
• Added type annotations to new arguments/methods/functions.
• Added an entry in the latest doc/source/whatsnew/vX.X.X.rst file if fixing a bug or adding a new feature.

Summary
Fix incorrect Series.isin results when comparing signed int64 values with uint64 values that are not equal. Previously, mixing signed and unsigned 64-bit integers could trigger a numeric common-type coercion to float64 which may lose precision and produce false positives. This change prevents that unsafe upcast by preferring an object-based comparison when signed and unsigned integer types are mixed.

Root cause
When isin attempted to find a common numeric dtype between comps (left side) and values (right side), mixing signed int64 with uint64 could lead to casting both sides to float64. Converting large 64-bit integers to float64 loses precision and can make two distinct integers compare as equal.

What I changed
[algorithms.py]:
Adjusted the condition used before converting values to an object array so that when dtypes differ and either side is an unsigned integer, [values] is converted to object (i.e., Python-level equality / hashtable lookup) instead of numeric coercion. This makes the mixed signed/unsigned decision symmetric and avoids unsafe float upcasts.
[test_isin.py]
Added test_isin_int64_vs_uint64_mismatch which reproduces the reported case and asserts the correct False result.