[Bugfix] Validate lora adapters to avoid crashing server

[joerunde]

This PR addresses an issue where loading an invalid lora adapter will crash the engine and shut the server down. This happens because dynamically loaded adapters are first loaded just-in-time, during model execution. Invalid adapters will raise at load time, and any exceptions raised during model execution will shut down the engine and the server.

Instead, this PR updates the /v1/load_lora_adapter handler to first call engine.add_lora to ensure that the adapter can be successfully loaded. On error, the adapter is not added to the list of available models to use. This should result in a better user experience, as users will immediately know if there is a problem with their adapter when they try to load it, instead of having it crash their server later when they try to use it.

FIX #11702

[github-actions]

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[jeejeelee]

Very useful feature! I will review this PR asap . Thanks!

[jeejeelee]

Thank you very much for your contribution, overall LGTM