frontend: CreateNamespaceButton: fix react-hooks/set-state-in-effect by deriving validation state during render

[Shahazadi-Shaguftha-Syed]

Summary

Fixes the react-hooks/set-state-in-effect violation in CreateNamespaceButton.tsx where validation state was set inside a useEffect on every namespaceName change. Validation is now derived directly during render instead.

Related Issue

Fixes #6120
Part of umbrella issue #5183

Changes

• Removed the useEffect that called setIsValidNamespaceName and setNameHelperMessage on every namespaceName change
• Derived isValidNamespaceFormat and formatHelperMessage directly during render
• Kept namespaceExistsError as state since it's set asynchronously from the 409 error handler
• Reset namespaceExistsError when the user edits the name field again

Steps to Test

1. Open the Create Namespace dialog
2. Type an invalid namespace name , error message should show immediately, no extra re-render
3. Try creating a namespace that already exists, should show "already exists" error
4. Edit the name again , error should clear and format validation should resume

Notes for the Reviewer

Behavior should be identical to before; this only changes how the validation state is computed (render-time derivation instead of effect-driven state).

[kubernetes-prow]

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[Shahazadi-Shaguftha-Syed]

@illume This fixes the react-hooks/set-state-in-effect violation in CreateNamespaceButton.tsx (issue #6120, part of #5183). Validation state is now derived during render instead of via useEffect. Would appreciate a review when you get a chance!