[BUGFIX] Process US regions without border countries

[Mirochill]

This PR...

• Keeps -nbc country runs for Geofabrik IDs like us/texas aligned with the tile names generated by CountryGeofabrik.
• Adds a regression expectation for us/texas so no-border processing uses us_texas, matching the split tile filenames.

Considerations and implementations

When border-country processing is disabled, CountryOsmData previously kept the raw wanted map id in border_countries. For US regions this meant us/texas, while CountryGeofabrik.find_needed_countries() normalizes tile country names to us_texas. The later split and merge stages compare those names, so no country split files were added to the Osmosis merge command.

This normalizes the no-border country key in the same way as the tile list and downloaded map filenames.

Fixes #288

How to test

1. Not run locally.
2. git diff --check

Pull Request Checklist

• Reviewed the Contributing Guidelines
  • Especially the Pull-Requests section
• Commits (and commit-messages) are understandable
• Tested with macOS / Linux
• Tested with Windows