refactor: replace direct slog usage with api.Logger in handlers

[sumitakhuli]

Description

Fixes #694

This PR refactors several REST API handlers to eliminate direct slog usages and replace them with the contextual api.Logger.

Previously, some handlers bypassed the structured logging configuration by calling slog.Warn(), slog.Info(), or slog.Error() directly, or by using slog.String rather than passing alternating key-value string arguments to the context logger. This change enforces consistency with other handlers (e.g., trips_for_route_handler.go) to ensure all log output routes properly through the *RestAPI receiver.

Changes Made

• trip_details_handler.go: Replaced direct slog.Warn events mapped to api.Logger.Warn. Removed slog.String wrappers in favor of variadic key-value arguments.
• report_problem_with_stop_handler.go & report_problem_with_trip_handler.go: Removed default slog.Default() fallbacks. Overhauled logging.LogOperation and logging.LogError logic to invoke corresponding .Info() and .Error() calls cleanly onto api.Logger.
• Test Files (trip_for_vehicle_handler_test.go, stops_for_route_handler_test.go): Swapped standard slog.New logger instantiations in favor of logging.NewStructuredLogger, dropping the log/slog imports across the rest api bounds.

Acceptance Criteria Met

• Audited all *_handler.go files in internal/restapi/ for direct slog usage.
• Replaced all direct slog.Warn(), slog.Info(), slog.Error() calls with the equivalent api.Logger.Warn(), api.Logger.Info(), api.Logger.Error() calls.
• Verified grep -r "slog\." internal/restapi/*handler*.go returns exactly zero matches.
• make test passes (confirmed logic & syntax integrity).
• make lint passes.

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