feat(image-generation): add StepFun provider support and StepPlan docs

- Add StepFunImageGenerationClient with step-image-edit-2 / step-1x-medium support
- Map aspect ratios to StepFun size strings (WxH order)
- Add style_reference for step-1x-medium reference-image generation
- Register in image gen provider registry (auto-discovered by nanobot.py)
- Add 7 unit tests: payload, default size, explicit size, style_reference (1x/non-1x), missing key, no-images
- Add StepFun section to docs/image-generation.md with provider config
- Add StepPlan (订阅制) subsection with apiBase override example

tests/providers/test_image_generation.py

@@ -14,6 +14,7 @@ from nanobot.providers.image_generation import (
     ImageGenerationError,
     MiniMaxImageGenerationClient,
     OpenRouterImageGenerationClient,
+    StepFunImageGenerationClient,
 )
 
 PNG_BYTES = (
@@ -387,3 +388,130 @@ async def test_minimax_payload_and_response_with_reference_image(tmp_path: Path)
     assert body["aspect_ratio"] == "21:9"
     assert body["subject_reference"][0]["type"] == "character"
     assert body["subject_reference"][0]["image_file"].startswith("data:image/png;base64,")
+
+
+# ---------------------------------------------------------------------------
+# StepFun (阶跃星辰)
+# ---------------------------------------------------------------------------
+
+@pytest.mark.asyncio
+async def test_stepfun_payload_and_response_with_aspect_ratio() -> None:
+    fake = FakeClient(FakeResponse({"data": [{"b64_json": RAW_B64}]}))
+    client = StepFunImageGenerationClient(
+        api_key="sk-sf-test",
+        api_base="https://api.stepfun.com/v1",
+        extra_headers={"X-Test": "1"},
+        client=fake,  # type: ignore[arg-type]
+    )
+
+    response = await client.generate(
+        prompt="a cat on the moon",
+        model="step-image-edit-2",
+        aspect_ratio="16:9",
+    )
+
+    assert response.images == [PNG_DATA_URL]
+    call = fake.calls[0]
+    assert call["url"] == "https://api.stepfun.com/v1/images/generations"
+    assert call["headers"]["Authorization"] == "Bearer sk-sf-test"
+    assert call["headers"]["X-Test"] == "1"
+    body = call["json"]
+    assert body["model"] == "step-image-edit-2"
+    assert body["prompt"] == "a cat on the moon"
+    assert body["response_format"] == "b64_json"
+    assert body["n"] == 1
+    assert body["size"] == "1280x800"
+
+
+@pytest.mark.asyncio
+async def test_stepfun_default_size_when_no_aspect_ratio() -> None:
+    fake = FakeClient(FakeResponse({"data": [{"b64_json": RAW_B64}]}))
+    client = StepFunImageGenerationClient(
+        api_key="sk-sf-test",
+        api_base="https://api.stepfun.com/v1",
+        client=fake,  # type: ignore[arg-type]
+    )
+
+    await client.generate(prompt="a dog", model="step-image-edit-2")
+
+    body = fake.calls[0]["json"]
+    assert body["size"] == "1024x1024"
+
+
+@pytest.mark.asyncio
+async def test_stepfun_uses_explicit_image_size() -> None:
+    fake = FakeClient(FakeResponse({"data": [{"b64_json": RAW_B64}]}))
+    client = StepFunImageGenerationClient(
+        api_key="sk-sf-test",
+        api_base="https://api.stepfun.com/v1",
+        client=fake,  # type: ignore[arg-type]
+    )
+
+    await client.generate(
+        prompt="a bird",
+        model="step-image-edit-2",
+        image_size="1024x1024",
+    )
+
+    body = fake.calls[0]["json"]
+    assert body["size"] == "1024x1024"
+
+
+@pytest.mark.asyncio
+async def test_stepfun_style_reference_on_1x_model(tmp_path: Path) -> None:
+    """step-1x-medium supports style_reference for reference-image generation."""
+    ref = tmp_path / "ref.png"
+    ref.write_bytes(PNG_BYTES)
+    fake = FakeClient(FakeResponse({"data": [{"b64_json": RAW_B64}]}))
+    client = StepFunImageGenerationClient(
+        api_key="sk-sf-test",
+        api_base="https://api.stepfun.com/v1",
+        client=fake,  # type: ignore[arg-type]
+    )
+
+    await client.generate(
+        prompt="in this style",
+        model="step-1x-medium",
+        reference_images=[str(ref)],
+    )
+
+    body = fake.calls[0]["json"]
+    assert "style_reference" in body
+    assert body["style_reference"]["source_url"].startswith("data:image/png;base64,")
+
+
+@pytest.mark.asyncio
+async def test_stepfun_no_style_reference_on_non_1x_model() -> None:
+    """step-image-edit-2 does not use style_reference; reference images are ignored."""
+    fake = FakeClient(FakeResponse({"data": [{"b64_json": RAW_B64}]}))
+    client = StepFunImageGenerationClient(
+        api_key="sk-sf-test",
+        api_base="https://api.stepfun.com/v1",
+        client=fake,  # type: ignore[arg-type]
+    )
+
+    await client.generate(
+        prompt="a flower",
+        model="step-image-edit-2",
+        reference_images=["/tmp/ref.png"],
+    )
+
+    body = fake.calls[0]["json"]
+    assert "style_reference" not in body
+
+
+@pytest.mark.asyncio
+async def test_stepfun_requires_api_key() -> None:
+    client = StepFunImageGenerationClient(api_key=None)
+
+    with pytest.raises(ImageGenerationError, match="API key"):
+        await client.generate(prompt="draw", model="step-image-edit-2")
+
+
+@pytest.mark.asyncio
+async def test_stepfun_no_images_raises() -> None:
+    fake = FakeClient(FakeResponse({"data": [{"text": "sorry"}]}))
+    client = StepFunImageGenerationClient(api_key="sk-sf-test", client=fake)  # type: ignore[arg-type]
+
+    with pytest.raises(ImageGenerationError, match="returned no images"):
+        await client.generate(prompt="draw", model="step-image-edit-2")