core: fallback for speed limit tags

[bougue-pe]

Only handling the speed for now, no work on output, warnings and logs. Extra-code to handle long-names currently used on speedLimitTag (both in infra or in requested simu).

also: little fix on Speed.toString()

Test:

• ✅ hand-tested locally that fallback works (for long-names) on small-infra: adding a speed section on Messagerie - ME100 somewhere else than South-East(to be able to select it), then using that speedLimitTag on a train to trigger fallback to MA100 on South-East.

Fix #7715

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 80.76923% with 15 lines in your changes missing coverage. Please review.

Project coverage is 28.43%. Comparing base (66dce06) to head (b907320). Report is 81 commits behind head on dev.

Files	Patch %	Lines
.../src/main/kotlin/fr/sncf/osrd/RawInfraRJSParser.kt	74.07%	4 Missing and 3 partials :warning:
...kotlin/fr/sncf/osrd/sim_infra/impl/RawInfraImpl.kt	84.78%	1 Missing and 6 partials :warning:
.../src/main/kotlin/fr/sncf/osrd/utils/units/Speed.kt	0.00%	1 Missing :warning:

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## dev #7903 +/- ## ============================================ - Coverage 28.67% 28.43% -0.24% - Complexity 2058 2075 +17 ============================================ Files 1249 1264 +15 Lines 154405 155174 +769 Branches 3040 3073 +33 ============================================ - Hits 44271 44123 -148 - Misses 108321 109222 +901 - Partials 1813 1829 +16 ``` | [Flag](https://app.codecov.io/gh/OpenRailAssociation/osrd/pull/7903/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/7903/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None) | `75.04% <80.76%> (+0.03%)` | :arrow_up: | | [editoast](https://app.codecov.io/gh/OpenRailAssociation/osrd/pull/7903/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None) | `71.56% <ø> (-0.37%)` | :arrow_down: | | [front](https://app.codecov.io/gh/OpenRailAssociation/osrd/pull/7903/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None) | `9.99% <ø> (-0.03%)` | :arrow_down: | | [gateway](https://app.codecov.io/gh/OpenRailAssociation/osrd/pull/7903/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None) | `2.34% <ø> (-0.07%)` | :arrow_down: | | [railjson_generator](https://app.codecov.io/gh/OpenRailAssociation/osrd/pull/7903/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/7903/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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