feat(community): add Brave Search web search tool (#3528)

* feat(community): add Brave Search web search tool

Add a community web_search provider backed by the official Brave Search
API (https://api.search.brave.com/res/v1/web/search). API key is read
from the tool config (inline api_key) or the BRAVE_SEARCH_API_KEY env
var. Output schema (title/url/content) matches existing search tools.
No new dependencies (uses the existing httpx). Also wires up the setup
wizard, doctor health check, config example, and EN/ZH docs.

* refactor(community): drop redundant [:count] slice in Brave search

The Brave API already caps results via the `count` request param, so
client-side slicing was redundant. Tests now simulate the API honoring
`count` instead of relying on the slice. Addresses PR review nit.

* style(tests): apply ruff format to test_doctor.py

Collapse multiline write_text calls onto single lines to satisfy the
CI ruff formatter (lint-backend was failing on format --check).

backend/docs/CONFIGURATION.md

@@ -234,7 +234,7 @@ tools:
 ```
 
 **Built-in Tools**:
-- `web_search` - Search the web (DuckDuckGo, Tavily, Exa, InfoQuest, Firecrawl)
+- `web_search` - Search the web (DuckDuckGo, Tavily, Brave, Exa, InfoQuest, Firecrawl)
 - `web_fetch` - Fetch web pages (Jina AI, Exa, InfoQuest, Firecrawl)
 - `ls` - List directory contents
 - `read_file` - Read file contents
@@ -364,6 +364,7 @@ models:
 - `MIMO_API_KEY` - Xiaomi MiMo API key
 - `NOVITA_API_KEY` - Novita API key (OpenAI-compatible endpoint)
 - `TAVILY_API_KEY` - Tavily search API key
+- `BRAVE_SEARCH_API_KEY` - Brave Search API key
 - `DEER_FLOW_PROJECT_ROOT` - Project root for relative runtime paths
 - `DEER_FLOW_CONFIG_PATH` - Custom config file path
 - `DEER_FLOW_EXTENSIONS_CONFIG_PATH` - Custom extensions config file path