Fix syntax error false positives for await outside functions (#21763)

## Summary

Fixes #21750 and a related bug in `PLE1142`. We were not properly
considering generators to be valid `await` contexts, which caused the
`F704` issue. One of the tests I added for this also uncovered an issue
in `PLE1142` for comprehensions nested within async generators because
we were only checking the current scope rather than traversing the
nested context.

## Test Plan

Both of these rules are implemented as semantic syntax errors, so I
added tests (and fixes) in both Ruff and ty.

crates/ruff_python_parser/tests/fixtures.rs

@@ -573,7 +573,7 @@ impl SemanticSyntaxContext for SemanticSyntaxCheckerVisitor<'_> {
         true
     }
 
-    fn in_generator_scope(&self) -> bool {
+    fn in_generator_context(&self) -> bool {
         true
     }