fix(test): wait for MariaDB with the root password, not the init window

The entrypoint installs root@localhost without unix_socket auth and sets
its password during init, so the passwordless socket probe only passed
when it landed in the window before SET PASSWORD. Missing that window
timed the e2e MariaDB suites out at random.

The probe now authenticates with the root password it already received
and requires @@skip_networking = 0, so the entrypoint's temporary server
no longer counts as ready. The anonymous-preemption bootstrap logs in
with the root password as well.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

tests/e2e/helpers/process.py

@@ -96,12 +96,13 @@ def wait_for_mariadb(
     container: str, *, root_password: str, timeout_s: int = 90
 ) -> None:
     """
-    Liveness probe for MariaDB.
+    Readiness probe for MariaDB, as root over the socket.
 
-    IMPORTANT (MariaDB 11):
-    Root TCP auth is often restricted (unix_socket auth), so a TCP ping like
-    `mariadb-admin -uroot -p... -h localhost ping` can fail even though the server is up.
-    We therefore check readiness via a socket-based query.
+    The entrypoint drops root@127.0.0.1, so root is socket-only, and it
+    installs root@localhost without unix_socket auth: a probe without the
+    password only passes in the init window before the password is set.
+    `@@skip_networking` must be 0 so the entrypoint's temporary server,
+    which shuts down right after init, does not count as ready.
     """
     deadline = time.time() + timeout_s
     while time.time() < deadline:
@@ -112,12 +113,15 @@ def wait_for_mariadb(
                 container,
                 "sh",
                 "-lc",
-                'mariadb -uroot --protocol=socket -e "SELECT 1;"',
+                (
+                    f"mariadb -uroot -p{root_password} --protocol=socket "
+                    '-N -e "SELECT @@skip_networking;"'
+                ),
             ],
             capture=True,
             check=False,
         )
-        if p.returncode == 0:
+        if p.returncode == 0 and p.stdout.strip() == "0":
             return
         time.sleep(1)
     raise TimeoutError(