core: handle ETCS braking curve with target after the end of the path

[bougue-pe]

Description and goal

After #10402 (comment)
Part of https://github.com/osrd-project/osrd-confidential/issues/948

Decide/explore what to do when braking curve's target is after the end of the path (for now, unknown path properties and so on).
Currently propagating the last properties at the final point of the path (which is not so conservative as the slope could drop before SvL).

Parts to edit/change:

• https://github.com/OpenRailAssociation/osrd/pull/10402/files#diff-0d80e29b28340d502e35311edcdb6ef92f2fb455a6bf53dc8bca9c7867ad1d74R583-R588
• https://github.com/OpenRailAssociation/osrd/pull/10402/files#diff-2f428c7de5cb96ae31dd848f1e290d5ce29c0ace7a60282c4f7e108a389c245fR114-R117

Acceptance criteria

TBD on what's implemented, postponed (TODO, issue), tracked and planned.