### What problem does this PR solve?
Closes#15608.
The ExeSQL agent tool (`agent/tools/exesql.py`) opens database
connections to a node-author-controlled host/port with no SSRF
validation. The sibling `test_db_connection` endpoint already validates
the host via `common.ssrf_guard.assert_host_is_safe` (added by PR
#14860), but the tool that actually performs the connection at agent run
time was left unguarded — so the guard is bypassed simply by running the
agent. An agent author can point the host at `127.0.0.1`,
`169.254.169.254` (cloud metadata), or any internal RFC1918 host/port,
turning ExeSQL into an internal port-scanner / metadata-fetch primitive.
### Fix
Mirror the accepted endpoint guard: validate (and resolve) the host
once, before the `db_type` dispatch, and connect to the validated public
IP so a later DNS change cannot rebind the host to an internal address.
- Add `from common.ssrf_guard import assert_host_is_safe`.
- `safe_host = assert_host_is_safe(self._param.host)` before the
dispatch (rejects loopback, link-local/metadata, RFC1918, and
unresolvable hosts).
- Substitute the validated IP into all 6 driver branches: mysql/mariadb,
oceanbase, postgres, mssql, trino, IBM DB2.
Adds `test/unit_test/agent/tools/test_exesql_ssrf.py` covering loopback,
link-local/metadata, RFC1918, and empty-host rejection (before any
connection), plus an allowed host dialing the validated IP.
### Validation
- `python3 -m py_compile agent/tools/exesql.py`
- `ruff check agent/tools/exesql.py
test/unit_test/agent/tools/test_exesql_ssrf.py`
- `pytest test/unit_test/agent/tools/test_exesql_ssrf.py` — 5 passed
### Type of change
- [x] Bug Fix (non-breaking change which fixes an issue)
---------
Co-authored-by: Zhichang Yu <yuzhichang@gmail.com>
