Add tests for gemini-3.1-flash-image-preview supports_reasoning

Verify supports_reasoning returns True for all three provider variants:
- base (gemini-3.1-flash-image-preview)
- gemini/ prefixed (via _supports_factory fallback)
- vertex_ai/ prefixed (via _supports_factory fallback)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

tests/test_litellm/llms/vertex_ai/gemini/test_gemini_3_1_flash_image_supports_reasoning.py

@@ -0,0 +1,60 @@
+"""
+Tests for gemini-3.1-flash-image-preview supports_reasoning flag.
+
+Verifies that supports_reasoning returns True for all provider variants,
+including the fallback from provider-prefixed entries to the base entry.
+"""
+
+import os
+
+import pytest
+
+import litellm
+from litellm.utils import supports_reasoning
+
+
+@pytest.fixture(autouse=True)
+def load_local_model_cost():
+    """Load model cost from local JSON so that uncommitted changes are picked up."""
+    original_env = os.getenv("LITELLM_LOCAL_MODEL_COST_MAP")
+    original_model_cost = getattr(litellm, "model_cost", None)
+
+    os.environ["LITELLM_LOCAL_MODEL_COST_MAP"] = "True"
+    litellm.model_cost = litellm.get_model_cost_map(url="")
+
+    yield
+
+    if original_env is None:
+        os.environ.pop("LITELLM_LOCAL_MODEL_COST_MAP", None)
+    else:
+        os.environ["LITELLM_LOCAL_MODEL_COST_MAP"] = original_env
+
+    if original_model_cost is not None:
+        litellm.model_cost = original_model_cost
+
+
+def test_gemini_3_1_flash_image_preview_supports_reasoning_base():
+    """Test that the base entry supports reasoning."""
+    assert supports_reasoning(model="gemini-3.1-flash-image-preview") is True
+
+
+def test_gemini_3_1_flash_image_preview_supports_reasoning_gemini_provider():
+    """Test that the gemini/ prefixed entry inherits supports_reasoning via fallback."""
+    assert (
+        supports_reasoning(
+            model="gemini/gemini-3.1-flash-image-preview",
+            custom_llm_provider="gemini",
+        )
+        is True
+    )
+
+
+def test_gemini_3_1_flash_image_preview_supports_reasoning_vertex_ai_provider():
+    """Test that the vertex_ai/ prefixed entry inherits supports_reasoning via fallback."""
+    assert (
+        supports_reasoning(
+            model="gemini-3.1-flash-image-preview",
+            custom_llm_provider="vertex_ai",
+        )
+        is True
+    )