Fix syntax error false positive on nested alternative patterns (#21104)

## Summary

Fixes #21101 by storing the child visitor's names in the parent visitor.
This makes sure that `visitor.names` on line 1818 isn't empty after we
visit a nested OR pattern.

## Test Plan

New inline test cases derived from the issue,
[playground](https://play.ruff.rs/7b6439ac-ee8f-4593-9a3e-c2aa34a595d0)

crates/ruff_python_parser/resources/inline/ok/nested_alternative_patterns.py

@@ -0,0 +1,7 @@
+match ruff:
+    case {"lint": {"select": x} | {"extend-select": x}} | {"select": x}:
+        ...
+match 42:
+    case [[x] | [x]] | x: ...
+match 42:
+    case [[x | x] | [x]] | x: ...