[Task] Admin SQL-dialect tests are H2-only in CI (root cause of the open dialect bugs)

## Description
Admin ships `application-{mysql,pg,og,ob,oracle}.yml` configs and JDBC drivers, but the test suite only ever runs against H2-in-MySQL-mode. No CI job runs the admin JUnit suite against real MySQL/PG/OpenGauss/Oracle. The only dialect exercise is `e2e-k8s.yml:152-162` (h2/mysql/postgres/opengauss compose) — slow e2e, not the unit suite, and does not cover Oracle/OceanBase.

## Location
```
.github/workflows/ci.yml:125-137 (clean test -Prelease)
shenyu-admin/src/test/resources/application.yml (profiles.active: h2)
shenyu-admin/pom.xml:181-266 (mysql/postgresql/opengauss/oracle drivers present but no test profile)
```

## Impact
This is the root-cause class for open bugs #6161, #6175, #6136, #6177, #5631, #5961 — dialect SQL divergences merge undetected.

## Suggested fix
Add a Testcontainers/docker-compose CI matrix running `shenyu-admin` tests with `spring.profiles.active=mysql|pg|og|oracle`; gate it.

## Related existing issue(s)
None

_Identified during the 2026-08-02 audit; full list in [`docs/issue-candidates-2026-08-02.md`](docs/issue-candidates-2026-08-02.md)._

Read .github/workflows/ci.yml:125-137, shenyu-admin/src/test/resources/application.yml, and shenyu-admin/pom.xml:181-266 to understand the current test command, active profile, and available drivers. Compare the existing e2e-k8s.yml:152-162 database setup, then verify that the admin JUnit suite runs for the requested database profiles in CI and that failures gate the workflow.

Written by the indexing model from the issue text.