Narrow mapping to dict for FixedDictStrategy in fixed_dictionaries

The public signature is now Mapping (covariant), but FixedDictStrategy
requires a concrete dict, which check_type already guarantees at runtime.
Cast to satisfy the package-wide mypy check.

https://claude.ai/code/session_01GGWSpKXv1sihrTexfxyxBo

Author: claude

hypothesis/src/hypothesis/strategies/_internal/core.py

@@ -584,7 +584,12 @@ def fixed_dictionaries(
                 f"which is invalid: {set(mapping) & set(optional)!r}"
             )
 
-    return FixedDictStrategy(mapping, optional=optional)
+    # check_type above guarantees both are dicts at runtime, but the public
+    # signature is Mapping (covariant) so we narrow for FixedDictStrategy here.
+    return FixedDictStrategy(
+        cast(dict[Any, SearchStrategy[Any]], mapping),
+        optional=cast(dict[Any, SearchStrategy[Any]] | None, optional),
+    )
 
 
 _get_first_item = operator.itemgetter(0)