fix(open-webui): drop MCP exposure and address remaining review comments

Per ajnart's review (and CodeRabbit withdrawing its own MCP comment on the
predecessor PR), the Open WebUI chat isn't a meaningful MCP surface:
- remove the .meta({ mcp }) metadata from every openWebUi procedure

Remaining CodeRabbit comments from the predecessor PR that weren't carried over:
- processWeb: constrain the URL to http(s) and reject loopback/metadata hosts
  (private LAN ranges stay allowed for self-hosters); it is also creator-gated
- uploadFile/transcribe: bound the base64 payload and validate filename/type
- docs: align the example URL with the integration default port (8080)
- new chat: clear the composer draft so it doesn't leak into the fresh chat
- web ingestion: keep the typed URL when ingestion fails
- persistence: fall back to the default title for image-only opening turns, and
  don't persist a partial reply from a failed turn as finished history
- multimodal round-trip: make the native chat/history contracts accept the same
  text-or-parts content union as completions, and flatten (not drop) non-text
  nodes when rebuilding transcripts so attachment-backed chats aren't lost.
  Image data URLs are still kept out of persisted history by design.

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

@@ -24,7 +24,7 @@ import { openWebUiWidget } from "../../widgets/open-webui";
 <AddingIntegration />
 
 :::info URL configuration
-Use the root URL of your Open WebUI instance, for example `http://localhost:3000`. Homarr calls the Open WebUI API under `/api` automatically.
+Use the root URL of your Open WebUI instance, for example `http://localhost:8080` (Open WebUI's default port). Homarr calls the Open WebUI API under `/api` automatically.
 :::
 
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