feat(image-generation): add Gemini provider support

Adds GeminiImageGenerationClient covering both Imagen 4 (:predict) and
Gemini Flash (:generateContent), wires the gemini ProviderConfig through
the SDK, API server, and gateway entry points, and updates the
image-generation docs and skill. Errors from the Gemini endpoints are
logged and surface with the HTTP status and parsed message instead of an
empty string.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tests/providers/test_image_generation.py

@@ -8,6 +8,7 @@ import pytest
 
 from nanobot.providers.image_generation import (
     AIHubMixImageGenerationClient,
+    GeminiImageGenerationClient,
     GeneratedImageResponse,
     ImageGenerationError,
     OpenRouterImageGenerationClient,
@@ -202,3 +203,137 @@ async def test_aihubmix_image_generation_downloads_url_response() -> None:
 
     assert response.images[0].startswith("data:image/png;base64,")
     assert fake.get_calls[0]["url"] == "https://cdn.example/image.png"
+
+
+RAW_B64 = PNG_DATA_URL.removeprefix("data:image/png;base64,")
+
+
+@pytest.mark.asyncio
+async def test_gemini_imagen_payload_and_response() -> None:
+    fake = FakeClient(
+        FakeResponse({"predictions": [{"bytesBase64Encoded": RAW_B64, "mimeType": "image/png"}]})
+    )
+    client = GeminiImageGenerationClient(
+        api_key="AIza-test",
+        api_base="https://generativelanguage.googleapis.com/v1beta",
+        client=fake,  # type: ignore[arg-type]
+    )
+
+    response = await client.generate(
+        prompt="a sunset",
+        model="imagen-4.0-generate-001",
+        aspect_ratio="16:9",
+    )
+
+    assert response.images == [PNG_DATA_URL]
+    assert response.content == ""
+    call = fake.calls[0]
+    assert call["url"].endswith(":predict")
+    assert call["headers"]["x-goog-api-key"] == "AIza-test"
+    assert "params" not in call
+    body = call["json"]
+    assert body["instances"] == [{"prompt": "a sunset"}]
+    assert body["parameters"]["sampleCount"] == 1
+    assert body["parameters"]["aspectRatio"] == "16:9"
+
+
+@pytest.mark.asyncio
+async def test_gemini_imagen_ignores_unsupported_aspect_ratio() -> None:
+    fake = FakeClient(
+        FakeResponse({"predictions": [{"bytesBase64Encoded": RAW_B64, "mimeType": "image/png"}]})
+    )
+    client = GeminiImageGenerationClient(api_key="AIza-test", client=fake)  # type: ignore[arg-type]
+
+    await client.generate(prompt="a sunset", model="imagen-4.0-generate-001", aspect_ratio="2:3")
+
+    body = fake.calls[0]["json"]
+    assert "aspectRatio" not in body["parameters"]
+
+
+@pytest.mark.asyncio
+async def test_gemini_flash_payload_and_response() -> None:
+    fake = FakeClient(
+        FakeResponse(
+            {
+                "candidates": [
+                    {
+                        "content": {
+                            "parts": [
+                                {"text": "here is your image"},
+                                {"inlineData": {"mimeType": "image/png", "data": RAW_B64}},
+                            ]
+                        }
+                    }
+                ]
+            }
+        )
+    )
+    client = GeminiImageGenerationClient(
+        api_key="AIza-test",
+        api_base="https://generativelanguage.googleapis.com/v1beta",
+        client=fake,  # type: ignore[arg-type]
+    )
+
+    response = await client.generate(
+        prompt="draw a cat",
+        model="gemini-2.0-flash-preview-image-generation",
+    )
+
+    assert response.images == [PNG_DATA_URL]
+    assert response.content == "here is your image"
+    call = fake.calls[0]
+    assert call["url"].endswith(":generateContent")
+    assert call["headers"]["x-goog-api-key"] == "AIza-test"
+    assert "params" not in call
+    body = call["json"]
+    assert body["generationConfig"]["responseModalities"] == ["TEXT", "IMAGE"]
+    assert body["contents"][0]["parts"][-1] == {"text": "draw a cat"}
+
+
+@pytest.mark.asyncio
+async def test_gemini_flash_reference_images(tmp_path: Path) -> None:
+    ref = tmp_path / "ref.png"
+    ref.write_bytes(PNG_BYTES)
+    fake = FakeClient(
+        FakeResponse(
+            {
+                "candidates": [
+                    {
+                        "content": {
+                            "parts": [{"inlineData": {"mimeType": "image/png", "data": RAW_B64}}]
+                        }
+                    }
+                ]
+            }
+        )
+    )
+    client = GeminiImageGenerationClient(api_key="AIza-test", client=fake)  # type: ignore[arg-type]
+
+    response = await client.generate(
+        prompt="edit this",
+        model="gemini-2.0-flash-preview-image-generation",
+        reference_images=[str(ref)],
+    )
+
+    assert response.images == [PNG_DATA_URL]
+    parts = fake.calls[0]["json"]["contents"][0]["parts"]
+    assert parts[0]["inlineData"]["mimeType"] == "image/png"
+    assert parts[0]["inlineData"]["data"].startswith("iVBOR")
+    assert parts[1] == {"text": "edit this"}
+
+
+@pytest.mark.asyncio
+async def test_gemini_requires_api_key() -> None:
+    client = GeminiImageGenerationClient(api_key=None)
+
+    with pytest.raises(ImageGenerationError, match="API key"):
+        await client.generate(prompt="draw", model="imagen-4.0-generate-001")
+
+
+@pytest.mark.asyncio
+async def test_gemini_no_images_raises() -> None:
+    fake = FakeClient(FakeResponse({"candidates": [{"content": {"parts": [{"text": "sorry"}]}}]}))
+    client = GeminiImageGenerationClient(api_key="AIza-test", client=fake)  # type: ignore[arg-type]
+
+    with pytest.raises(ImageGenerationError, match="returned no images"):
+        await client.generate(prompt="draw", model="gemini-2.0-flash-preview-image-generation")