core: add error when stdcm requests don't end with a stop

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 33.33333% with 2 lines in your changes missing coverage. Please review.

Project coverage is 28.48%. Comparing base (e0f4f41) to head (67ede2b).

Files	Patch %	Lines
...n/fr/sncf/osrd/api/api_v2/stdcm/STDCMEndpointV2.kt	0.00%	2 Missing :warning:

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## dev #7919 +/- ## ============================================ - Coverage 28.49% 28.48% -0.01% Complexity 2059 2059 ============================================ Files 1254 1254 Lines 154467 154469 +2 Branches 3047 3047 ============================================ - Hits 44011 43999 -12 - Misses 108641 108655 +14 Partials 1815 1815 ``` | [Flag](https://app.codecov.io/gh/OpenRailAssociation/osrd/pull/7919/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None) | Coverage Δ | | |---|---|---| | [core](https://app.codecov.io/gh/OpenRailAssociation/osrd/pull/7919/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None) | `75.00% <33.33%> (-0.01%)` | :arrow_down: | | [editoast](https://app.codecov.io/gh/OpenRailAssociation/osrd/pull/7919/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None) | `71.64% <ø> (-0.05%)` | :arrow_down: | | [front](https://app.codecov.io/gh/OpenRailAssociation/osrd/pull/7919/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None) | `9.97% <ø> (ø)` | | | [gateway](https://app.codecov.io/gh/OpenRailAssociation/osrd/pull/7919/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None) | `2.34% <ø> (ø)` | | | [railjson_generator](https://app.codecov.io/gh/OpenRailAssociation/osrd/pull/7919/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None) | `87.49% <ø> (ø)` | | | [tests](https://app.codecov.io/gh/OpenRailAssociation/osrd/pull/7919/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None) | `72.93% <ø> (ø)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None#carryforward-flags-in-the-pull-request-comment) to find out more.

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