Request form: .gov domains screen reader output for form validation

[AnnaGingle]

Story

As a user who relies on screen readers for navigation,
I want to interact with forms that clearly communicate input fields, required information, and error messages,
so that I can complete the form independently and accurately without confusion or frustration.

Acceptance Criteria

For all form fields on the .gov domain page, associate each alert using accessibility best practices:

• Success state: That domain is available! We’ll try to give you the domain you want, but it's not guaranteed. After you complete this form, we’ll evaluate whether your request meets our requirements.
• Error state: That domain isn’t available. [Read more about choosing your .gov domain](https://get.gov/domains/choosing).
• Validation error: Enter the .gov domain you want without any periods.
• The text inputs are compliant with WCAG 2.1 SC 4.1.2 and Trusted Tester 5.C by properly associating the accessible name, description, and alert messages for each field.

Additional Context

On the .gov domain page each alert isn’t connected to the form field. This fails 5.C programmatic label and WCAG SC 4.1.2

5.C requires that the combination of the accessible name, accessible description, and other programmatic associations (e.g., table column and/or row associations) describes each input field and includes all relevant instructions and cues (textual and graphical).

• Grouping controls W3C
• Using the fieldset and legend elements GOV.UK
• ANDI Guide
• Trusted Tester docs
  

Issue Links

🔄 Relates to: #2872

Prioritization Comments

WCAG Conformance Level: 2.1 AA
WCAG Standard that applies:
4.1.2 - Name, Role, Value (Critical)
3.3.1 - Error Identification (Critical)
3.3.2 - Labels or Instructions (Critical)
Critical Failure if Not Met?: Yes
Prioritization: Blocker
Reason: Properly associating form field alerts is crucial for screen reader users, as it enables them to accurately understand input requirements, validation errors, and success states. Without this, screen reader users may not perceive critical information about form completion, which severely hinders their ability to submit forms independently and accurately.