HDS-5263: `Alert` composition change for discussion

[MelSumner]

:pushpin: Summary

If merged, this PR re-factors the HTML of the Alert so that the alert/aria-live div are always in the DOM if the component is used.

This would resolve one issue where we had to play around with the positioning of the error conditional in order to get the component to work correctly in the helios-demo app while using assistive tech.

However, since this may have other implications that we may not be aware of, we're only submitting this one small change in order to facilitate a discussion about other details we may not know about yet.

We should probably figure out a way to trigger an error or some other alert-triggering condition on our showcase pages so that we can manually test in a more appropriate way.

:hammer_and_wrench: Detailed description

:camera_flash: Screenshots

:link: External links

Jira ticket: HDS-5263 Figma file: [if it applies]

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:eyes: Component checklist

• [ ] Percy was checked for any visual regression
• [ ] A changelog entry was added via Changesets if needed (see templates here)

:speech_balloon: Please consider using conventional comments when reviewing this PR.

:clipboard: PCI review checklist

• [ ] If applicable, I've documented a plan to revert these changes if they require more than reverting the pull request.
• [ ] If applicable, I've worked with GRC to document the impact of any changes to security controls. Examples of changes to controls include access controls, encryption, logging, etc.
• [ ] If applicable, I've worked with GRC to ensure compliance due to a significant change to the in-scope PCI environment. Examples include changes to operating systems, ports, protocols, services, cryptography-related components, PII processing code, etc.

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