fix: use native validation for duplicate file names

[mvanhorn]

• Resolves: [Bug]: Two different ' this name is already in use ' message implementations in file creation on web. #62686

Summary

Show the duplicate-name (and other filename) error through the platform validation popup instead of the NcTextField helper. setCustomValidity still blocks Create and Enter-key submit. The helper/error bindings are dropped so the same string is not shown twice.

Same pattern as FileEntryName and PublicPageMenuExternalDialog.

Fixes #62686

[susnux]

Blocking for a design decision on the issue first.

[mvanhorn]

@susnux holding until the design call lands.

This PR currently does option 1: keep the NcTextField helper and drop the proactive reportValidity() call so the native tooltip doesn't double up. Enter-key submit stays blocked via setCustomValidity.

On the issue, @kra-mo leaned native if the tooltip is aligned correctly. That's the opposite of this patch. Happy to invert it (native reporting, drop the custom helper) or leave this as-is once you pick a lane.

[mvanhorn]

@susnux inverted this to option 2 after @kra-mo's lean on the issue.

reportValidity() is back, and the NcTextField no longer binds :error / :helper-text to the same string, so you get the native tooltip only. Create stays disabled and Enter still fails native validation. Same pattern as FileEntryName and PublicPageMenuExternalDialog.

Happy to flip back if designers land on the helper instead.

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