Fix TiDB CI flakiness by waiting for server readiness by AmlanDalai · Pull Request #1292 · sqlancer/sqlancer

Partially addresses #1282.

The TiDB CI jobs were failing intermittently with:
ERROR 2013 (HY000): Lost connection to MySQL server at 'reading initial communication packet'

This indicates that the TiDB playground container had started but was not yet ready to accept MySQL connections when the tests began.

This PR stabilizes the TiDB CI by:

• pinning the TiDB playground to a stable image instead of a nightly build
• adding a readiness check that waits until the server is available before running tests

No changes were made to the SQLancer test logic, this fix is limited to CI setup.