test(jdbc): make PG integration tests self-contained on env-configured port

Restore 3 v2 tests lost in file corruption (persistNextStates roundtrip,
legacy columns all-null, schd_gen last_message_id CAS) — 13 JDBC cases.
Add idempotent Flyway migrate in @BeforeEach so an empty container works.
Parameterize compose port as ${MSGX_PG_PORT:-5432}. Verified 14/14 green
(0 skipped) against real PostgreSQL on :5433.

compose.yaml

@@ -40,7 +40,7 @@ services:
       POSTGRES_USER: ${MSGX_PG_USER:-msgx_dev}
       POSTGRES_PASSWORD: ${MSGX_PG_PASSWORD:-msgx_dev_pass}
     ports:
-      - "5432:5432"
+      - "${MSGX_PG_PORT:-5432}:5432"
     volumes:
       - pg_data:/var/lib/postgresql/data
     healthcheck: