[test] Fix unstable tests

[juntaozhang]

Purpose

fix unstable ut:
https://github.com/apache/paimon/actions/runs/29978032986/job/89113858219?pr=8819
https://github.com/apache/paimon/actions/runs/29979989905/job/89119609484?pr=8574

Tests

[JingsongLi]

• When the time is exactly ss=00, ms=000, LocalDateTime.toString() outputs 2026-07-23T12:34, but the current regex still enforces :ss, so the test will still fail with an extremely low probability; I have reproduced this locally.
• SparkMultimodalITCase simply changes the fixed port from 9092 to 9093; essentially, this merely shifts the port conflict. It is recommended to use a dynamic idle port.

[JingsongLi]

+1