fix(forms): make the JobStart age/disability gate answerable

[phrogwrld]

Refs #2199 ("If I click no. I cant type in the free text box but its mandatory").

Problem

JobStart Plus gates applicants aged 25+ who have no disability or long-term
health condition. That gate was implemented as a disabled, required text
field in the disability-support step:

{
  "ref": "components/generic-text",
  "overrides": {
    "isDisabled": true,
    "label": "Are you 25 years or older, without a disability?",
    "validations": {
      "required": { "value": true, "error": "If you are over 24, you must have a disability..." },
      "equal": { "error": "If you are over 24, you must have a disability..." }
    },
    "behaviours": [
      { "type": "fieldConditionalOn", "targetFieldId": "applicant-dob",
        "operator": "gte", "value": "25", "transform": "yearsSince" },
      { "type": "optionalIf", "targetFieldId": "has-disability", "value": "yes" }
    ]
  }
}

Three defects in one element:

1. Unsatisfiable. A 25+ applicant answering "no" saw a greyed-out box they
   could not type into, blocking Continue with no way forward.
2. No fieldId. It fell back to the registry default (generic-text), so it
   submitted under a junk key and would collide with any other defaulted
   generic-text field in the step.
3. Dead equal rule. An error with no value can never fire, so only
   required ever did anything.

Fix

Same gate, expressed the way this recipe already gates eligibility 40 lines away
(currently-employed): a radio carrying equal. Visibility, copy and policy are
unchanged — the applicant can now answer, and gets the authored message.

Verified against the real hydrated contract with the production validator
(@govtech-bb/form-validation):

Answer	Result
"Yes" (25+, no disability)	blocked — "If you are over 24, you must have a disability or long-term health condition to apply."
"No"	passes
unanswered	required error (only when visible, i.e. 25+ and has-disability != yes)

optionalIf relaxes only required (validation-builder.ts,
form-conditions/index.ts), so a conditionally-applied equal is not
expressible — which is why the gate stays a separate age-scoped control rather
than moving onto has-disability.

Guard

recipe-invariants.spec.ts now rejects any disabled-and-required field across all
76 recipes, with a negative test proving it bites. This was the only occurrence.

Verification

• nx run api:test — 1277 pass (was 1276; the new negative test)
• nx run api:build — green
• tsc -b — clean
• jobstart-plus-programme.smoke.spec.ts needs no change: it uses a 21-year-old,
  so the gate stays hidden

Note

Kept the hard block, since the recipe's own hint states the policy ("If you are
over 24, you must have a disability or long-term health condition to apply"). If
the intent is instead to let these applicants submit and be filtered later, say
so — that's a one-line change to drop the equal rule.

[amazon-inspector-n-virginia]

⏳ I'm reviewing this pull request for security vulnerabilities and code quality issues. I'll provide an update when I'm done

[github-actions]

Amplify PR previews

App	Status	URL
forms	⏭ Not affected	—
landing	⏭ Not affected	—
chat	⏭ Not affected	—
form-builder	⏭ Not affected	—
analytics	⏭ Not affected	—

Commit: e76979e · Branches auto-delete when this PR closes.

ℹ️ feature-flagging uses a single shared preview slot at a fixed URL (preview.feature-flagging.sandbox.alpha.gov.bb) — GitHub login works, but only one FF preview exists at a time (the most recent FF PR build wins). It reads/writes sandbox flag data.

[amazon-inspector-n-virginia]

✅ I finished the code review, and didn't find any security or code quality issues.