chore(tests): remove unused _batch_image helper from test_coverage_gaps.py

_batch_image() was defined in tests/test_coverage_gaps.py but never called
in that file — an identical helper already exists in test_image_to_text_node.py
where it is actively used. Dead code with no callers; safe to remove.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

tests/test_coverage_gaps.py

@@ -164,18 +164,6 @@ def _single_image() -> FakeTensor:
     return FakeTensor(np.array([[[1.0, 0.0, 0.0]]], dtype=np.float32))
 
 
-def _batch_image() -> FakeTensor:
-    return FakeTensor(
-        np.array(
-            [
-                [[[1.0, 0.0, 0.0]]],
-                [[[0.0, 0.0, 1.0]]],
-            ],
-            dtype=np.float32,
-        )
-    )
-
-
 def test_responses_kwargs_no_system_prompt_omits_instructions() -> None:
     """
     When system_prompt is blank the 'instructions' key must NOT appear in