Fix column requirements banner flicker

[Creylay]

Summary

Fixes the input/output column requirements banner in the "Prepare Dataset" step of session creation. It used to vanish silently (no message at all) whenever Input or Output Columns was emptied, and could briefly flash an incorrect red "invalid" state right after a dataset finished loading, even when the auto-selected columns were actually valid. Cleared fields now show an inline "Required" error, and the requirements banner reflects the real validation state without flickering.

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Type of Change

Check all that apply like this [x]:

• Backend change
• Frontend change
• CI / Workflow change
• Build / Packaging change
• Bug fix
• Documentation

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Changes (by file)

• DashAI/front/src/components/models/modelSession/PrepareDatasetStep.jsx:
  • Wired the existing (previously unused) inputError/outputError/inputHelperText/outputHelperText props on DivideDatasetColumns, so an empty Input/Output Columns field is marked red with a "Required" message instead of the whole requirements banner just disappearing.
  • Fixed the validation-controlling effect: when there are no columns selected, validationPending now resolves to false immediately (we already know the selection is invalid, no backend call needed) instead of getting stuck at true forever, which was hiding the banner permanently after a column was cleared.
  • Switched that effect from useEffect to useLayoutEffect to close a render race: when columns go from empty back to a valid selection (e.g. right after the dataset finishes loading and defaults are auto-selected), the pending flag now flips back to true synchronously before paint, instead of one commit too late, which previously caused a one-frame flash of an incorrect red "invalid" banner over already-valid columns.
  • Simplified columnsReady from a useState + useEffect pair into a plain derived value, removing an extra render cycle of lag.

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Testing

• Verified clearing the only Input/Output column now shows an inline red "Required" state on the field, while the requirements banner stays visible in its "invalid" state (instead of disappearing).