feat(image-gen): add image-to-image / editing to image_generate (#48705)

* feat(image-gen): add image-to-image / editing to image_generate

Brings image generation to parity with video generation: the unified
image_generate tool now edits/transforms a source image (image-to-image)
when given image_url / reference_image_urls, routing to each backend's
edit endpoint, exactly as video_generate routes to image-to-video.

- ImageGenProvider ABC: generate() gains keyword-only image_url +
  reference_image_urls; new capabilities() declares modalities +
  max_reference_images (defaults to text-only, backward compatible).
  success_response gains a modality field; adds normalize_reference_images.
- image_generate tool: schema exposes image_url + reference_image_urls;
  dynamic schema reflects the active model's actual edit capability so the
  agent knows when image_url is honored. Handler + plugin dispatch forward
  the new inputs; legacy/text-only providers get a clear modality_unsupported
  error instead of silently dropping the source image.
- In-tree FAL: 7 models gain edit endpoints (flux-2-klein, flux-2-pro,
  nano-banana-pro, gpt-image-1.5, gpt-image-2, ideogram/v3, qwen-image)
  with per-model edit_supports whitelists + reference caps; routes to the
  /edit endpoint and skips the upscaler for edits.
- Plugins: openai (images.edit, 16 refs), xai (/v1/images/edits via
  grok-imagine-image-quality, JSON body per xAI docs), krea
  (image_style_references, 10 refs). openai-codex stays text-only and
  rejects edits with an actionable error.
- Tests: 15 new (payload, routing, dispatch forwarding, dynamic schema,
  capabilities); updated 2 change-detector/lambda tests for the new schema.
- Docs: image-generation feature page, image-gen provider plugin guide,
  tools reference.

* fix(image-gen): preserve legacy passthrough in fal/krea plugin tests

Two existing plugin tests asserted pre-image-to-image behavior:
- fal: forward image_url/reference_image_urls only when supplied, so a
  text-to-image delegation stays byte-identical (no None kwargs).
- krea: keep dict-shaped image_style_references refs verbatim (the unified
  string refs go through normalize_reference_images; legacy non-string ref
  objects pass through unchanged) — fixes KeyError when callers pass the
  richer Krea ref-object shape.

* fix(image-gen): clearer not-capable message for text-to-image-only models

When a text-to-image-only model (incl. gpt-image-2 on the Codex OAuth path,
which can't do editing through the Responses image_generation tool) gets a
source image, say 'this model is not capable of image-to-image / editing —
provide a text-only prompt' rather than sending the user shopping for other
backends. Applies to the openai-codex guard, the in-tree FAL no-edit-endpoint
error, and the dynamic tool-schema text-only line.

website/docs/reference/tools-reference.md

@@ -114,7 +114,7 @@ Scoped to the Feishu document-comment handler. Drives comment read/write operati
 
 | Tool | Description | Requires environment |
 |------|-------------|----------------------|
-| `image_generate` | Generate high-quality images from text prompts using FAL.ai. The underlying model is user-configured (default: FLUX 2 Klein 9B, sub-1s generation) and is not selectable by the agent. Returns a single image URL. Display it using… | FAL_KEY |
+| `image_generate` | Generate images from text prompts (text-to-image) or edit/transform an existing image (image-to-image) via the user-configured backend (FAL.ai, OpenAI, xAI, Krea). Pass `image_url` to edit an image and `reference_image_urls` for style references; omit both for text-to-image. The model is user-configured and not selectable by the agent. Returns a single image URL or local path. | FAL_KEY / OPENAI_API_KEY / xAI OAuth / KREA_API_KEY |
 
 ## `kanban` toolset