feat(chat): add You.com web search provider (#18478)

### Summary

Adds You.com as a built-in Web Search provider for RAGFlow Chat,
alongside Tavily and Querit, using the provider-neutral dispatch #17813
put in place. No changes to existing Tavily or Querit behaviour.

You.com runs its own web index and returns several extracted passages
per result rather than a single meta description, so retrieved chunks
arrive with usable context.

---------

Co-authored-by: Brian Sparker <brainsparker@users.noreply.github.com>
Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>

docs/references/http_api_reference.md

@@ -3032,10 +3032,11 @@ curl --request POST \
   - `"use_kg"`: `boolean`
   - `"reasoning"`: `boolean`
   - `"cross_languages"`: `list[string]`
-  - `"web_search_provider"`: `string` The web search service to use. Supported values are `"tavily"`, `"querit"`, and `"serply"`. Defaults to `"tavily"` when omitted.
+  - `"web_search_provider"`: `string` The web search service to use. Supported values are `"tavily"`, `"querit"`, `"serply"`, and `"youcom"`. If omitted, Tavily is selected only when `"tavily_api_key"` is configured; otherwise web search is disabled.
   - `"tavily_api_key"`: `string`
   - `"querit_api_key"`: `string` The Querit API key. Set `web_search_provider` to `"querit"` when using this field.
   - `"serply_api_key"`: `string` The [Serply](https://serply.io) API key. Set `web_search_provider` to `"serply"` when using this field. See the [Serply documentation](https://serply.io/docs) for details.
+  - `"youcom_api_key"`: `string` The You.com API key. Set `web_search_provider` to `"youcom"` when using this field. Optional: You.com serves a rate-limited keyless endpoint, so `"youcom"` works with this field omitted, and a key lifts those limits.
   - `"toc_enhance"`: `boolean`
 - `"similarity_threshold"`: (*Body parameter*), `float`
 - `"vector_similarity_weight"`: (*Body parameter*), `float`