[ty] fix incorrect lazy scope narrowing (#19744)

## Summary

This is a follow-up to #19321.

Narrowing constraints introduced in a class scope were not applied even
when they can be applied in lazy nested scopes. This PR fixes so that
they are now applied.
Conversely, there were cases where narrowing constraints were being
applied in places where they should not, so it is also fixed.

## Test Plan

Some TODOs in `narrow/conditionals/nested.md` are now work correctly.

crates/ty_python_semantic/src/semantic_index/builder.rs

@@ -356,7 +356,9 @@ impl<'db, 'ast> SemanticIndexBuilder<'db, 'ast> {
             for nested_symbol in self.place_tables[popped_scope_id].symbols() {
                 // For the same reason, symbols declared as nonlocal or global are not recorded.
                 // Also, if the enclosing scope allows its members to be modified from elsewhere, the snapshot will not be recorded.
-                if self.scopes[enclosing_scope_id].visibility().is_public() {
+                // (In the case of class scopes, class variables can be modified from elsewhere, but this has no effect in nested scopes,
+                // as class variables are not visible to them)
+                if self.scopes[enclosing_scope_id].kind().is_module() {
                     continue;
                 }