Fix local backend trial cleanup

[VincentG1234]

Summary

• improve local backend cleanup by tracking trial controllers alongside futures
• request cooperative cancellation before forcing resource cleanup
• wait briefly for running local trials to stop before clearing backend state

Why

The previous local cleanup path only cancelled futures, which is not enough when trial work is already running. This change makes shutdown and cleanup more reliable by propagating cancellation to the trial controller and attempting resource cleanup for still-running trials.

[VincentG1234]

@hgsmn tell me If that fix the bug you met