fix(image): narrow Gemini Flash aspect-ratio and image-size scoping

Address review feedback that the capability checks were broader than the
documented per-model matrix:

- Drop the extreme aspect ratios (1:4, 4:1, 1:8, 8:1) from the Flash
  allow-list. They are only documented for 3.1 Flash / Flash Lite, so the
  global set could send an unsupported ratio to 2.5 Flash Image or 3.1 Pro
  Image. Keep the ratios common to every Flash image model.
- Identify imageSize support positively via "gemini-3" instead of excluding
  "2.5". The old predicate also matched gemini-2.0-flash-preview-image-
  generation, which (with the default 1K size) altered that model's request
  shape even though only Gemini 3+ image models accept a configurable size.

Add tests for the gemini-2.0 image-size drop and the extreme-ratio drop.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/providers/test_image_generation.py

@@ -465,14 +465,32 @@ async def test_gemini_flash_2_5_drops_image_size() -> None:
 
 
 @pytest.mark.asyncio
-async def test_gemini_flash_ignores_unsupported_hints() -> None:
+async def test_gemini_flash_2_0_drops_image_size() -> None:
     fake = FakeClient(_gemini_flash_image_response())
     client = GeminiImageGenerationClient(api_key="AIza-test", client=fake)  # type: ignore[arg-type]
 
+    await client.generate(
+        prompt="draw a cat",
+        model="gemini-2.0-flash-preview-image-generation",
+        aspect_ratio="16:9",
+        image_size="1K",
+    )
+
+    image_config = fake.calls[0]["json"]["generationConfig"]["responseFormat"]["image"]
+    assert image_config == {"aspectRatio": "16:9"}
+
+
+@pytest.mark.asyncio
+async def test_gemini_flash_ignores_unsupported_hints() -> None:
+    fake = FakeClient(_gemini_flash_image_response())
+    client = GeminiImageGenerationClient(api_key="AIza-test", client=fake)  # type: ignore[arg-type]
+
+    # 7:5 is not a documented ratio; 1:8 is only valid for 3.1 Flash, not Pro;
+    # 1024x1024 is not a valid Gemini image-size token. All are dropped.
     await client.generate(
         prompt="draw a cat",
         model="gemini-3-pro-image",
-        aspect_ratio="7:5",
+        aspect_ratio="1:8",
         image_size="1024x1024",
     )