Add form validation to AccountBanner for account selection

[JulienVannier66]

What does this PR do?

Add form validation logic to the Utils::AccountBanner component to handle account selection from the side panel.

• Integrate OSS::Form in the subtitle area to register a required form field (account.select)
• Conditionally display the required marker (*) and validation modifiers based on @selectedAccount presence
• Expose an @onFormSetup callback so consumers can access the form instance
• Handle error state (red border + message) via a getter derived from @selectedAccount and form errors

What are the observable changes?

Screencast.from.2026-04-02.17-31-05.webm

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• Feel free to migrate existing components to Glimmer Components.
• Write new ones exclusively in it.

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Good PR checklist

• Title makes sense
• Is against the correct branch
• Only addresses one issue
• Properly assigned
• Added/updated tests
• Added/updated documentation
• Migrated touched components to Glimmer Components
• Properly labeled

[linear]

VEL-7480 Handle account selection from payment app's side panel + form validation

[olxmpe]

You showed me how it works (and it works perfectly), but I still think it's really weird to put an OSS::Form here. Couldn't we just pass a “required” flag and and a feedback message as arguments and handle the form logic in the parent component, where register-form-field would be used on the account-banner component instead ?