LMR: consist change might output that a path exists when it doesn't

[Erashin]

What happened?

Right now, when trying a path that backtracks WITH a consist change (A - B - A for ex), 2 preliminary validation pathfinding requests are sent:

• one for A - B
• one for B - A

Problem: if the path is impossible without a backtrack, the path will be validated here BUT stdcm won't be able to find a path. Especially if B is a passage via where no backtrack is allowed 😂

What did you expect to happen?

We need a pretty consecuent change: the backend pathfinding is ready to have a consist change has an input, but we have to modify the front-end/editoast to only send one pathfinding request for the full path, then plug it in correctly in core.
All that to say: only one pathfinding request should be sent, with the can_backtrack AND consist change information.

How can we reproduce it (as minimally and precisely as possible)?

1. Go to LMR
2. Create a correct backtracking path, but the via should be a passage
3. The validation pathfinding finds a solution
4. Launch stdcm: it won't work

On which environments the bug occurs?

Dev (SNCF), Local

On which browser the bug occurs?

Firefox

OSRD version (top right corner Account button > Informations)

3578eb3