fix: treat NaN as empty in form schema so required validation fires for cleared number inputs

[serikjensen]

Summary

react-aria's NumberField emits NaN when its input is cleared. In buildFormSchema, the makeOptional preprocess only normalized '' and null to undefined, so NaN slipped through to inner field preprocessors like coerceNaN(0) and was silently coerced to 0 — bypassing the superRefine required check (isEmpty(0) is false). Result: required currency fields looked required (no (optional) label) but the form happily submitted with no value.

This was most visible in useFederalTaxesForm, where every non-filingStatus field is a number with 0 as a legitimate value, so the domain-level workaround payScheduleSchema uses (a 1–31 range check) wasn't applicable. useCompensationForm was also affected — clearing the rate field reported RATE_MINIMUM instead of REQUIRED.

Changes

• src/partner-hook-utils/form/buildFormSchema.ts: isEmpty now also recognizes NaN, and makeOptional reuses isEmpty so cleared numeric inputs normalize to undefined before any inner preprocessor runs. The required check then fires correctly.
• src/partner-hook-utils/form/buildFormSchema.test.ts: framework-level regression coverage for the NaN-required and NaN-optional cases.
• src/components/Employee/FederalTaxes/shared/useFederalTaxesForm/federalTaxesSchema.test.ts (new): domain coverage matching the original repro — extraWithholding: NaN with optionalFieldsToRequire.update set must produce REQUIRED, while 0 remains a valid value.
• src/components/Employee/Compensation/shared/useCompensationForm/useCompensationForm.test.tsx: the test that previously asserted RATE_MINIMUM for NaN rate now asserts REQUIRED, matching the user-facing expectation when an input is empty.

Test plan

• npm run test -- --run src/partner-hook-utils src/components/Company/PaySchedule src/components/Employee/FederalTaxes src/components/Employee/Compensation — 268 passing
• npm run test -- --run (full suite) — 2470 passing
• Manual: clear Step 4c: Extra withholding on the Federal Taxes page and submit — should now block submission with the configured REQUIRED validation message instead of silently coercing to 0

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