Refactor GenTL CTI discovery and loading

Introduce a dedicated gentl_discovery utility to locate .cti GenTL producer files (from explicit files, env vars, glob patterns and extra dirs) and add selection helpers. Refactor GenTLCameraBackend to use the new discovery logic: resolve multiple CTIs, load all viable producers, persist CTI diagnostics into settings (cti_files, cti_files_loaded, cti_files_failed), and provide robust class-level Harvester building for discovery/quick_ping/get_device_count/rebind_settings. Remove legacy glob/search helpers and change behavior to prefer explicit user config while failing only when no producers can be found/loaded. Update tests and fixtures to use tmp_path-created dummy .cti files, adjust patched Harvester setup, and add unit tests covering CTI discovery and selection policies (including newest/raise-if-multiple behavior).

Author: C-Achard