refactor(gtfs): replace direct stderr printing with structured logging

[3rabiii]

Description

This PR refactors GetVehicleForTrip in gtfs_manager.go to use the project's structured logger (slog / logging.LogError) instead of writing directly to os.stderr using fmt.Fprintf.

Changes

• Replaced fmt.Fprintf(os.Stderr, ...) with logging.LogError.
• Added structured context fields (e.g., trip_id, block_id) to the logs for better observability.
• Removed unused os import.

@aaronbrethorst
fixes : #300

[aaronbrethorst]

Hey Adel, thanks for tackling this logging cleanup — replacing raw stderr writes with structured logging is exactly the kind of improvement that pays off in observability. The structured context fields (trip_id, block_id) are a nice touch. Before we can merge this, I need you to fix one issue:

Critical

1. Nil error panic in LogError (internal/gtfs/gtfs_manager.go:351-353): The condition on line 351 is err != nil || !requestedTrip.BlockID.Valid, which means err can be nil when the block ID is simply invalid. logging.LogError unconditionally calls err.Error() (structured_logging.go:30), so passing a nil error will cause a nil pointer dereference panic. The original fmt.Fprintf handled this gracefully because %v prints <nil> for nil values. You'll need to either guard against nil before calling LogError, or split the condition into two separate checks with different log messages (which would also be more informative — "trip not found" vs. "trip has no block ID" are different situations worth distinguishing).

Thanks again, and I look forward to merging this change.

[3rabiii]

Thanks for the critical catch! I've addressed the potential nil pointer panic in GetVehicleForTrip:

1. Split the Condition: Refactored the logic to check err != nil separately from !requestedTrip.BlockID.Valid.
2. Safe Logging: logging.LogError is now guarded and only called for actual DB errors.
3. Improved Semantics: Missing Block IDs are now treated as Debug info rather than errors, as suggested.

Ready for review!

@aaronbrethorst
fixes : #300