feat(open-webui): add image, knowledge and webpage attachments with retry

Extend the AI Chat widget with three ways to ground a conversation:

- Images: attach images to a message, sent inline as OpenAI multimodal
  content parts for vision-capable models. A hint warns when the selected
  model likely has no vision support.
- Knowledge bases: pick RAG collections exposed by /api/v1/knowledge/.
- Web pages: ingest a URL via /api/v1/retrieval/process/web.

Open WebUI does not run RAG from the chat-completions `files` parameter,
so attached collections (web pages and knowledge bases) are grounded by
retrieving the most relevant chunks at send time
(/api/v1/retrieval/query/collection) and injecting them as system context.

Also add a Retry action to re-run the last turn after a model failure, and
handle the paginated `{ items, total }` shape of the knowledge endpoint on
newer Open WebUI versions.

apps/docs/docs/widgets/open-webui/index.mdx

@@ -13,7 +13,7 @@ import { openWebUiIntegration } from "@site/docs/integrations/open-webui";
 
 <WidgetHeader widget={openWebUiWidget} categories={["AI", "Chat"]} />
 
-The AI Chat widget lets you chat with AI models served by Open WebUI directly from your dashboard. Responses stream in token-by-token, you can pick any model your Open WebUI instance exposes (Ollama, OpenAI, and other configured providers), and you can browse and reopen your Open WebUI chat history.
+The AI Chat widget lets you chat with AI models served by Open WebUI directly from your dashboard. Responses stream in token-by-token, you can pick any model your Open WebUI instance exposes (Ollama, OpenAI, and other configured providers), and you can browse and reopen your Open WebUI chat history. You can also ground a conversation by attaching images, knowledge bases, or web pages.
 
 ### Supported Integrations
 
@@ -40,8 +40,13 @@ The AI Chat widget lets you chat with AI models served by Open WebUI directly fr
 | **Model picker**        | Lists every model exposed by Open WebUI's `/api/models` endpoint, selectable from the chat input. The first model is selected by default. |
 | **Streaming responses** | Assistant replies stream in live as they are generated.                                                                                   |
 | **Chat history**        | Browse and reopen previous chats. New conversations are saved back to Open WebUI.                                                         |
+| **Image attachments**   | Attach images to a message for vision-capable models (sent inline as multimodal content).                                                 |
+| **Knowledge bases**     | Ground replies on any RAG knowledge collection (`/api/v1/knowledge/`) your account owns.                                                   |
+| **Web pages**           | Attach a URL and Open WebUI fetches and embeds the page as context for the reply.                                                          |
 
 ### Notes
 
 - The widget uses a single Open WebUI integration.
 - Conversations are stored in Open WebUI itself, under the account that owns the API key, so they also appear in the Open WebUI app.
+- Knowledge bases and attached web pages stay active for the whole conversation until you remove them; images apply only to the message you attach them to.
+- Conversation text is saved to Open WebUI history, but attached image data is not persisted.