Fix: add SSRF guard for agent test_db_connection endpoint (#14860)

### What problem does this PR solve?

Closes #14858

The `test_db_connection` endpoint in the agent API accepts a
user-supplied `host` and connects to it directly via database drivers
(MySQL/PostgreSQL) without any validation. This allows an attacker to
probe internal network addresses (e.g. `127.0.0.1`, `10.x.x.x`,
link-local, etc.) through the server — a classic Server-Side Request
Forgery (SSRF) vulnerability.

This PR adds an SSRF guard that resolves the host and rejects any
address that is not globally routable before the database connection is
attempted.

**Changes:**
- **`common/ssrf_guard.py`** — Added `assert_host_is_safe()`, a
host-level counterpart of the existing `assert_url_is_safe()`, designed
for non-HTTP protocols (database drivers) where there is no URL to
parse.
- **`api/apps/restful_apis/agent_api.py`** — Call
`assert_host_is_safe(req["host"])` at the top of `test_db_connection` so
that non-public hosts are rejected early with a clear error message.

Fixes #14858

### Type of change

- [x] Bug Fix (non-breaking change which fixes an issue)

---------

Co-authored-by: Jin Hai <haijin.chn@gmail.com>

api/apps/restful_apis/agent_api.py

@@ -60,6 +60,7 @@ from api.utils.api_utils import (
     validate_request,
 )
 from common import settings
+from common.ssrf_guard import assert_host_is_safe
 from common.constants import RetCode
 from common.misc_utils import get_uuid, thread_pool_exec
 from peewee import MySQLDatabase, PostgresqlDatabase
@@ -782,12 +783,27 @@ async def rerun_agent(tenant_id):
 @login_required
 async def test_db_connection():
     req = await get_request_json()
+    try:
+        safe_host = assert_host_is_safe(req["host"])
+    except ValueError as exc:
+        logging.warning(
+            "Rejected test_db_connection: unsafe host %r (db_type=%s, user=%s): %s",
+            req.get("host"), req.get("db_type"), current_user.id, exc,
+        )
+        return get_data_error_result(message=str(exc))
+    except OSError as exc:
+        logging.warning(
+            "Rejected test_db_connection: cannot resolve host %r (db_type=%s, user=%s): %s",
+            req.get("host"), req.get("db_type"), current_user.id, exc,
+        )
+        logging.debug("Full resolver exception for host %r", req.get("host"), exc_info=True)
+        return get_data_error_result(message=f"Could not resolve host {req.get('host')!r}.")
     try:
         if req["db_type"] in ["mysql", "mariadb"]:
             db = MySQLDatabase(
                 req["database"],
                 user=req["username"],
-                host=req["host"],
+                host=safe_host,
                 port=req["port"],
                 password=req["password"],
             )
@@ -797,7 +813,7 @@ async def test_db_connection():
             db = MySQLDatabase(
                 req["database"],
                 user=req["username"],
-                host=req["host"],
+                host=safe_host,
                 port=req["port"],
                 password=req["password"],
                 charset="utf8mb4",
@@ -808,7 +824,7 @@ async def test_db_connection():
             db = PostgresqlDatabase(
                 req["database"],
                 user=req["username"],
-                host=req["host"],
+                host=safe_host,
                 port=req["port"],
                 password=req["password"],
             )
@@ -819,7 +835,7 @@ async def test_db_connection():
 
             connection_string = (
                 f"DRIVER={{ODBC Driver 17 for SQL Server}};"
-                f"SERVER={req['host']},{req['port']};"
+                f"SERVER={safe_host},{req['port']};"
                 f"DATABASE={req['database']};"
                 f"UID={req['username']};"
                 f"PWD={req['password']};"
@@ -838,7 +854,7 @@ async def test_db_connection():
 
             conn_str = (
                 f"DATABASE={req['database']};"
-                f"HOSTNAME={req['host']};"
+                f"HOSTNAME={safe_host};"
                 f"PORT={req['port']};"
                 f"PROTOCOL=TCPIP;"
                 f"UID={req['username']};"
@@ -847,7 +863,7 @@ async def test_db_connection():
             logging.info(
                 "DATABASE=%s;HOSTNAME=%s;PORT=%s;PROTOCOL=TCPIP;UID=%s;PWD=****;",
                 req["database"],
-                req["host"],
+                safe_host,
                 req["port"],
                 req["username"],
             )
@@ -873,7 +889,7 @@ async def test_db_connection():
                 auth = trino.BasicAuthentication(req.get("username") or "ragflow", req["password"])
 
             conn = trino.dbapi.connect(
-                host=req["host"],
+                host=safe_host,
                 port=int(req["port"] or 8080),
                 user=req["username"] or "ragflow",
                 catalog=catalog,