fix: support redirect as string

[cka-y]

Summary:
This pull request includes a change to the process_redirects method in the api/src/scripts/populate_db_gtfs.py file. A try-except block is added to handle ValueError exceptions when converting mdb_source_id to a float. This ensures that if the conversion fails, the mdb_source_id is stripped and used as-is.

Successful run: https://github.com/MobilityData/mobility-feed-api/actions/runs/13142416339
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