editoast: implement /level_crossing_occupancy endpoint logic

[aschantraine]

Implements full logic for /level_crossing_occupancy/ endpoint assuming all trains are PacedTrain:

• Move interpolate_arrival_time() from track_occupancy.rs to projection.rs to be reused for level_crossing_occupancy
• Add track_offset() method to LevelCrossing schema
• Implement complete occupancy calculation workflow:
  • Load infra, level crossings and trains
  • Run train pathfinding and simulation
  • Compute occupancy time windows
  • Group results by level crossing ID
  • Skip trains with missing rolling stock, failed simulations, no pedal intersection, or that do not fully traverse the short zone
• Add i18n messages for level_crossing errors in en and fr
• Add tests

This only works for PacedTrain as long as the TrainSchedule migration has not been completed.

[github-actions]

⚠️ API changes

This Pull Request introduces some changes in the API:

• please own it: notify or even prepare dedicated PR(s) to consumer projects

Changes in Schemas

Dear @aschantraine, please ensure the following:

• changes should match in osrd_schemas and editoast schemas
• the RailJSON version in osrd_schemas::infra.py and editoast schemas::infra::railjson.rs should match and be bumped on any change of infra schemas (same goes for rolling-stock between osrd_schemas::rolling_stock.py and editoast schemas::rolling_stock.rs)
• the version of the Python package in osrd_schemas::pyproject.toml should be bumped on any change