[flake8-return] Fix false-positive for variables used inside nested functions in RET504 (#18433)

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## Summary

<!-- What's the purpose of the change? What does it do, and why? -->
This PR is the same as #17656.

I accidentally deleted the branch of that PR, so I'm creating a new one.

Fixes #14052

## Test Plan

Add regression tests
<!-- How was it tested? -->

crates/ruff_python_semantic/src/model.rs

@@ -2094,6 +2094,20 @@ impl<'a> SemanticModel<'a> {
             None
         })
     }
+
+    /// Finds and returns the [`Scope`] corresponding to a given [`ast::StmtFunctionDef`].
+    ///
+    /// This method searches all scopes created by a function definition, comparing the
+    /// [`TextRange`] of the provided `function_def` with the the range of the function
+    /// associated with the scope.
+    pub fn function_scope(&self, function_def: &ast::StmtFunctionDef) -> Option<&Scope> {
+        self.scopes.iter().find(|scope| {
+            let Some(function) = scope.kind.as_function() else {
+                return false;
+            };
+            function.range() == function_def.range()
+        })
+    }
 }
 
 pub struct ShadowedBinding {