fix: exclude loop variables from test import fallback

Make get_referenced_names_from_source scope-aware by reusing
UndefinedNameCollector, preventing invalid imports like `i` and `v`
from loop variables in AI-generated tests.

django/aiservice/testgen/postprocessing/add_missing_imports.py

@@ -269,18 +269,17 @@ def get_imports_from_source_code(source_code: str) -> dict[str, str]:
 
 
 def get_referenced_names_from_source(source_code: str) -> set[str]:
-    """Get all names referenced (used) in source code.
+    """Get names that are used but not locally defined in source code.
 
-    This captures names that appear as identifiers in the source, even if they're
-    not explicitly defined or imported in the visible snippet. Useful for finding
-    symbols that the source module uses but whose definition may be missing from
-    the snippet.
+    This returns names that could potentially be module-level exports,
+    excluding locally-scoped names like loop variables, function parameters,
+    and comprehension variables.
 
     Args:
         source_code: The source code to parse
 
     Returns:
-        Set of all names referenced in the source code
+        Set of names that are used but not defined locally
 
     """
     try:
@@ -288,21 +287,10 @@ def get_referenced_names_from_source(source_code: str) -> set[str]:
     except SyntaxError:
         return set()
 
-    names: set[str] = set()
-    # Use iterative stack-based traversal instead of ast.walk() generator
-    stack = [tree]
-    while stack:
-        node = stack.pop()
-        if node.__class__ is ast.Name:
-            names.add(node.id)
-        # Directly traverse child nodes using _fields
-        for field in node._fields:
-            value = getattr(node, field, None)
-            if isinstance(value, list):
-                stack.extend(reversed([item for item in value if isinstance(item, ast.AST)]))
-            elif isinstance(value, ast.AST):
-                stack.append(value)
-    return names
+    collector = UndefinedNameCollector()
+    collector.visit(tree)
+    # Return names that are used but not defined locally
+    return collector.used_names - collector.defined_names - collector.imported_names
 
 
 def get_local_definitions(test_code: str) -> set[str]: