fix(DF-832): source GOV.UK Pay email from CYA confirmation field

[mokhld]

Proposed change

DF-832 requires the email pre-populated on GOV.UK Pay to come from the CYA confirmation email field, not a designer-configured form question. This change reads state.userConfirmationEmailAddress instead of state[options.emailField].

Jira ticket:

Type of change

• Bug fix
• New feature
• Breaking change
• Misc. (documentation, build updates, etc)

Checklist

• You have executed this code locally and it performs as expected.
• You have added tests to verify your code works.
• You have added code comments and JSDoc, where appropriate.
• There is no commented-out code.
• You have added developer docs in `README.md` and `docs/*` (where appropriate, e.g. new features).
• The tests are passing (`npm run test`).
• The linting checks are passing (`npm run lint`).
• The code has been formatted (`npm run format`).

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
100.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud