feat: add OpenSERP self-hosted web search backend (#27437)

Add self-hosted OpenSERP as a web search engine option. OpenSERP
provides browser-rendered search across Google, Bing, Yandex, Baidu,
DuckDuckGo, and Ecosia with no API keys required.

- New module: retrieval/web/openserp.py (async, uses aiohttp session pool)
- Config: OPENSERP_BASE_URL env var (defaults to http://localhost:7070)
- Routing: search_web() dispatch for 'openserp' engine
- Follows existing patterns (searxng, brave)

Co-authored-by: crustopher-lgtm <crustopher-lgtm@users.noreply.github.com>

backend/open_webui/retrieval/web/openserp.py

@@ -0,0 +1,45 @@
+from __future__ import annotations
+
+import logging
+
+from open_webui.retrieval.web.main import SearchResult, get_filtered_results
+from open_webui.utils.session_pool import get_session
+
+log = logging.getLogger(__name__)
+
+
+async def search_openserp(
+    base_url: str,
+    query: str,
+    count: int,
+    filter_list: list[str | None] | None = None,
+) -> list[SearchResult]:
+    """Query an OpenSERP instance and return normalised results.
+
+    OpenSERP aggregates results from 6 engines (google, bing, yandex,
+    baidu, duckduckgo, ecosia) at once via ``/mega/search``.
+
+    No API key is required -- only a reachable OpenSERP base URL.
+    """
+    url = f"{base_url.rstrip('/')}/mega/search"
+    params = {"text": query, "limit": count}
+
+    log.debug("searching OpenSERP at %s", url)
+
+    session = await get_session()
+    async with session.get(url, params=params) as response:
+        response.raise_for_status()
+        payload = await response.json()
+
+    results = payload.get("results", [])
+    if filter_list:
+        results = get_filtered_results(results, filter_list)
+
+    return [
+        SearchResult(
+            link=item.get("url", ""),
+            title=item.get("title"),
+            snippet=item.get("snippet"),
+        )
+        for item in results[:count]
+    ]