test(gemini): de-flake test_gemini_image_size_limit_exceeded

Mock the image fetch instead of downloading a 50MB+ image from
upload.wikimedia.org. The runner was intermittently rate-limited
(HTTP 429), so the code raised "Unable to fetch image ... Status
code: 429" and the size-limit assertions failed even though
pytest.raises(litellm.ImageFetchError) still matched.

Mirror the established LargeImageClient pattern in
tests/test_litellm/litellm_core_utils/test_image_handling.py: stub
litellm.module_level_client with a response whose Content-Length
exceeds the 50MB limit and bypass SSRF validation, so the
size-limit rejection path is exercised deterministically with no
external network dependency.

tests/llm_translation/test_gemini.py

@@ -1594,13 +1594,46 @@ def test_gemini_31_flash_lite_reasoning_effort_minimal():
     ), "gemini-3.1-flash-lite-preview should use thinkingLevel, not thinkingBudget"
 
 
-def test_gemini_image_size_limit_exceeded():
+def test_gemini_image_size_limit_exceeded(monkeypatch):
     """
     Test that large images exceeding MAX_IMAGE_URL_DOWNLOAD_SIZE_MB are rejected.
 
     This validates that the 50MB default limit prevents downloading very large images
     that could cause memory issues and pod crashes.
+
+    The image fetch is mocked (mirroring the LargeImageClient pattern in
+    tests/test_litellm/litellm_core_utils/test_image_handling.py) so the test
+    deterministically exercises the size-limit rejection path without any
+    external network dependency.
     """
+    from httpx import Request, Response
+
+    from litellm.litellm_core_utils.prompt_templates import image_handling
+
+    class LargeImageClient:
+        """Returns a response whose Content-Length exceeds the 50MB limit."""
+
+        def get(self, url, follow_redirects=True):
+            size_bytes = int(100 * 1024 * 1024)  # 100MB > 50MB default limit
+            return Response(
+                status_code=200,
+                headers={
+                    "Content-Type": "image/jpeg",
+                    "Content-Length": str(size_bytes),
+                },
+                content=b"x" * size_bytes,
+                request=Request("GET", url),
+            )
+
+    # Bypass SSRF validation (which would resolve DNS / hit the network) and
+    # route straight to our mocked client.
+    monkeypatch.setattr(
+        image_handling,
+        "safe_get",
+        lambda client, url, **kw: client.get(url, follow_redirects=True),
+    )
+    monkeypatch.setattr(litellm, "module_level_client", LargeImageClient())
+
     messages = [
         {
             "role": "user",
@@ -1608,7 +1641,7 @@ def test_gemini_image_size_limit_exceeded():
                 {"type": "text", "text": "What is in this image?"},
                 {
                     "type": "image_url",
-                    "image_url": "https://upload.wikimedia.org/wikipedia/commons/5/51/Blue_Marble_2002.jpg",
+                    "image_url": "https://example.com/large-image.jpg",
                 },
             ],
         }