fix(trip-service): resolve race condition in distributed trip assignment lock

[Ama2352]

Fixed critical race condition causing HTTP 500 errors under high concurrency (200 concurrent drivers). The issue occurred when JPA's optimistic locking exception was thrown at transaction commit time, outside the try-catch block.

Root Cause:

• tripRepository.save() defers DB write until transaction commit
• Transaction commits AFTER method returns (outside try-catch)
• StaleObjectStateException thrown at commit time escapes exception handler
• Results in HTTP 500 instead of graceful 409 Conflict

Solution:

• Changed save() to saveAndFlush() to force immediate DB write
• Exception now caught within try-catch block
• Returns AcceptResult.ALREADY_ASSIGNED (409 Conflict) gracefully
• Added @slf4j and warning log for optimistic lock conflicts
• Simplified DriverController to forward responses without redundant error handling