[ML] Job validation doesn't check lower bound on model memory limit

[elasticmachine]

Original comment by @droberts195:

The lowest permitted model memory limit is 1MB. The backend job creation endpoint will refuse to create a job with a lower model memory limit.

The UI currently does not check for this, so if you try to create a job with a limit of 1023KB you get an error from the backend endpoint:

But clicking the "Validate Job" button in this situation gives you a tick for "Model memory limit":

The 1MB minimum is the clear-cut check that should be added to the UI validation. However, there is another soft check that could also be added: if the model memory estimator is calculating a value based on a formula rather than a simple default then warn if the value chosen by the user is less than half this amount, as the job is almost certain to then hit hard limit.

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Part of #18368