Fix: Ensure type safety for OSM tags and IDs

[rodrigost23]

This PR introduces a few necessary fixes to prevent crashes during the GTFS to OSM conversion process, specifically regarding tag types and handling responses from the Overpass API.

Changes proposed in this pull request:

• Enforce string types for OSM tags: Added dictionary comprehensions to ensure all keys and values are strictly cast to strings, while also stripping out any None values. This prevents downstream crashes when generating the OSM XML, which expects string-only tags.
• Robust Overpass API node handling: Safely skip elements if the Overpass API returns a node without an ID, preventing KeyError crashes, and explicitly cast the node_id to an int.