fix(api): guard missing row in SearchService.get_detail (#15622)

## Summary
`SearchService.get_detail` crashed with `AttributeError` (HTTP 500) when
no matching row existed, because it called `.first().to_dict()` before
the `if not search` guard — making that guard dead code.

## Root cause
`.first()` returns `None` when the query matches nothing (deleted search
app, or joined `User` not `VALID`). `None.to_dict()` raises before the
guard runs.

## Fix
```diff
             .first()
-            .to_dict()
         )
         if not search:
             return {}
-        return search
+        return search.to_dict()
```
Guard the `None` first, then serialize — restoring the intended `{}`
"not found" return that every caller (`search_api`, `bot_api`,
`chat_api`, `dataset_api_service`) already handles.

## Files changed
- `api/db/services/search_service.py`

## Verification
- `ruff check` — clean
- Logic: `.first()` → `None` now hits `return {}` instead of
`None.to_dict()`. Local full pytest not run (heavy RAG deps); CI
validates.

## Note
Implemented with LLM assistance (model: claude-opus-4-8).

Closes #15621

Co-authored-by: dearsishs <MCarter112116@outlook.com>
Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

api/db/services/search_service.py

@@ -72,11 +72,10 @@ class SearchService(CommonService):
             .join(User, on=((User.id == cls.model.tenant_id) & (User.status == StatusEnum.VALID.value)))
             .where((cls.model.id == search_id) & (cls.model.status == StatusEnum.VALID.value))
             .first()
-            .to_dict()
         )
         if not search:
             return {}
-        return search
+        return search.to_dict()
 
     @classmethod
     @DB.connection_context()