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>

common/ssrf_guard.py

@@ -170,3 +170,42 @@ def assert_url_is_safe(
         raise ValueError(f"Hostname {hostname!r} resolved to no addresses.")
 
     return hostname, resolved_ip
+
+
+def assert_host_is_safe(host: str) -> str:
+    """Raise ``ValueError`` if *host* resolves to a non-public IP (SSRF guard for raw host/port connections).
+
+    This is the host-level counterpart of :func:`assert_url_is_safe`, intended
+    for callers that connect via database drivers or other non-HTTP protocols
+    where there is no URL to parse.
+
+    Returns the first validated public IP string so the caller can pin it if needed.
+    """
+    if not host:
+        raise ValueError("Host must not be empty.")
+
+    try:
+        addr_infos = socket.getaddrinfo(host, None)
+    except socket.gaierror as exc:
+        logger.warning("SSRF guard could not resolve host=%r reason=%s", host, exc)
+        raise ValueError(f"Could not resolve host {host!r}: {exc}") from exc
+
+    resolved_ip: str | None = None
+    for _family, _type, _proto, _canonname, sockaddr in addr_infos:
+        raw_ip = ipaddress.ip_address(sockaddr[0])
+        eff_ip = _effective_ip(raw_ip)
+        if not eff_ip.is_global:
+            logger.warning(
+                "SSRF guard blocked host: host=%r resolved to non-public address=%s",
+                host,
+                raw_ip,
+            )
+            raise ValueError(f"Host resolves to a non-public address ({raw_ip}), which is not allowed.")
+        if resolved_ip is None:
+            resolved_ip = str(raw_ip)
+
+    if resolved_ip is None:
+        logger.warning("SSRF guard blocked host: host=%r resolved to no addresses", host)
+        raise ValueError(f"Host {host!r} resolved to no addresses.")
+
+    return resolved_ip