feat: Validate split name while typing

[cephalization]

Resolves #9981

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Note

Validate split name on change and pluralize the "Create Split" header based on selected example count.

• Frontend:
  • app/src/components/datasetSplit/NewDatasetSplitForm.tsx: Enable real-time validation by setting useForm mode to "onChange".
  • app/src/pages/examples/ExamplesSplitMenu.tsx: Pluralize "example(s)" in the "Create Split" heading based on selectedExampleIds.length.

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