fix: make demographics race "Decline to respond" option exclusive

[youhaowei]

Issue

Closes #2884

On the application demographics step, the race question allowed "Decline to respond" to be selected at the same time as other race options. It should be mutually exclusive — the same exclusive-checkbox behavior already used in the application preferences/programs step.

Change

sites/public/src/pages/applications/review/demographics.tsx:

• Checking "Decline to respond" clears every other race option.
• Checking any other race option clears "Decline to respond".
• Reuses the shared setExclusive helper (shared-helpers/.../multiselectQuestions.tsx) that powers the same behavior in ApplicationMultiselectQuestionStep. The per-field onChange is wired through FieldGroup's existing inputProps — FieldGroup itself is unchanged.

Testing

• New unit tests in demographics.test.tsx cover both directions of exclusivity; full demographics suite is 14/14 passing.
• Manually verified in a browser on the running demographics step — checking "Decline to respond" clears selected race options, and checking another option clears "Decline to respond".
• tsc, ESLint, and Prettier are clean.

Known issue — not fixed here, needs discussion

FieldGroup (in @bloom-housing/ui-components) decides sub-option visibility from an internal click-tracked array (checkedInputs) rather than from form state. Because this fix unchecks options programmatically via setValue (which fires no click event), a parent option's sub-options stay visually expanded after the parent is cleared by the exclusive logic — e.g. expand "Asian", then check "Decline to respond": "Asian" and its sub-options uncheck correctly, but the sub-option rows remain on screen until the next interaction.

• Checked state and submitted data are correct — this is purely visual.
• It only occurs on the programmatic uncheck path; a normal user click still collapses sub-options correctly.
• Root cause is in ui-components FieldGroup and affects every consumer. The proper fix is to derive sub-option visibility from watched form state instead of the click-tracked mirror. That is a cross-repo change and is left for team discussion on how best to handle.

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[youhaowei]

Re-triggering CI — the failures are in submit-application.spec.ts (unhandled auth-client error at the address step, before demographics) and map-address-popup (map view flake). Both are unrelated to this PR's changes. Re-running.