azure: Add async validation for storage account name step

[nturinski]

According to the big C, this was the issue:

I've identified the root cause. The @microsoft/vscode-azext-utils library wraps validateInput with validOnTimeoutOrException which has a 2-second timeout — if the API call takes longer, validation silently returns undefined (valid), and for blob/file/queue/table there's no duplicate check at all.

So moving to the asyncValidate call should fix this.

Summary

Splits the storage account name validation in StorageAccountNameStep into synchronous and asynchronous parts:

• validateStorageAccountNameSync — Performs instant client-side checks (length constraints, invalid characters) so the user gets immediate feedback while typing.
• validateStorageAccountNameAvailability — Calls checkNameAvailability on the storage management client asynchronously via the new asyncValidationTask input box option, so the availability check no longer blocks the synchronous validation path.

This improves the responsiveness of the storage account name input box by deferring the network call to the async validation task.

[Copilot]

Pull request overview

This PR improves the UX responsiveness of the storage account name prompt by splitting validation into immediate synchronous checks and a deferred async availability check, using asyncValidationTask to avoid blocking the typing/validation loop with a network call.

Changes:

• Split storage account name validation into validateStorageAccountNameSync (local rules) and validateStorageAccountNameAvailability (ARM checkNameAvailability).
• Wire validateInput to the sync validator and asyncValidationTask to the availability check in the input box prompt.

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