Fail fast for closed pool health checks

[ittaigolde]

Fixes #2325.

The Dropwizard connectivity health check keeps a direct HikariPool reference. After HikariDataSource.close(), calling the health check still went through pool.getConnection(checkTimeoutMs), so it waited for the configured health-check timeout even though the pool was already shut down and could never provide a connection.

This change makes ConnectivityHealthCheck return an unhealthy result immediately when the pool is already shut down.

Reproduction before the fix:

• CodahaleMetricsTest#testConnectivityHealthCheckFailsFastAfterShutdown waited 765ms with connectivityCheckTimeoutMs=750.

Verification after the fix:

• JDK 21.0.11: mvn -Djacoco.skip=true -Dtest=CodahaleMetricsTest test
• JDK 25.0.3: mvn -Djacoco.skip=true -Dtest=CodahaleMetricsTest test