fix: Preserve narrowing in chained-comparison operands

A regression was introduced that caused loss of type inference for operands in chained comparisons, such as `None is not a == b`. The logic to prevent unwanted type widening was too aggressive and discarded valid narrowing from `is not` checks.

The fix adjusts the condition in `update_from_options` to allow updates when only a single frame is being merged. This correctly identifies sequential updates within expressions (like chained comparisons) and preserves the narrowing, while still preventing unwanted widening when merging multiple control-flow branches.

Author: aryanjsingh

mypy/binder.py

@@ -336,7 +336,12 @@ def update_from_options(self, frames: list[Frame]) -> bool:
                 # See check-isinstance.test::testNoneCheckDoesNotMakeTypeVarOptional
                 # This is a safe assumption: the fact that we checked something with `is`
                 # or `isinstance` does not change the type of the value.
-                continue
+                if len(frames) == 1:
+                    # This is likely a sequential update, not a merge of
+                    # conditional branches. Let the update proceed to preserve narrowing.
+                    pass
+                else:
+                    continue
 
             # Remove exact duplicates to save pointless work later, this is
             # a micro-optimization for --allow-redefinition-new.