test(billing): add unit tests for findValidPaymentMethod function

- 11 new tests covering empty array, single valid/invalid, mixed lists
- Tests for first valid selection, order preservation, link vs card priority
- Verifies non-mutation of input array
- Total: 36 tests for auto-topup-helpers (was 25, now 36)

Author: brandonkachen

packages/billing/src/__tests__/auto-topup-helpers.test.ts

@@ -2,6 +2,7 @@ import { describe, expect, it } from 'bun:test'
 
 import {
   filterValidPaymentMethods,
+  findValidPaymentMethod,
   isValidPaymentMethod,
 } from '../auto-topup-helpers'
 
@@ -271,4 +272,128 @@ describe('auto-topup-helpers', () => {
       expect(original[1].id).toBe('pm_expired')
     })
   })
+
+  describe('findValidPaymentMethod', () => {
+    it('should return undefined for empty array', () => {
+      const result = findValidPaymentMethod([])
+      expect(result).toBeUndefined()
+    })
+
+    it('should return the payment method when single valid card exists', () => {
+      const validCard = createCardPaymentMethod('pm_valid', 2099, 12)
+
+      const result = findValidPaymentMethod([validCard])
+
+      expect(result).toBeDefined()
+      expect(result?.id).toBe('pm_valid')
+    })
+
+    it('should return the payment method when single valid link exists', () => {
+      const validLink = createLinkPaymentMethod('pm_link')
+
+      const result = findValidPaymentMethod([validLink])
+
+      expect(result).toBeDefined()
+      expect(result?.id).toBe('pm_link')
+    })
+
+    it('should return undefined when single payment method is invalid', () => {
+      const expiredCard = createCardPaymentMethod('pm_expired', 2020, 1)
+
+      const result = findValidPaymentMethod([expiredCard])
+
+      expect(result).toBeUndefined()
+    })
+
+    it('should return undefined when all payment methods are invalid', () => {
+      const expiredCard1 = createCardPaymentMethod('pm_expired_1', 2020, 1)
+      const expiredCard2 = createCardPaymentMethod('pm_expired_2', 2015, 6)
+      const sepa = createPaymentMethodWithType('pm_sepa', 'sepa_debit')
+
+      const result = findValidPaymentMethod([expiredCard1, expiredCard2, sepa])
+
+      expect(result).toBeUndefined()
+    })
+
+    it('should return the first valid payment method from a mixed list', () => {
+      const expiredCard = createCardPaymentMethod('pm_expired', 2020, 1)
+      const validCard = createCardPaymentMethod('pm_valid', 2099, 12)
+      const validLink = createLinkPaymentMethod('pm_link')
+
+      const result = findValidPaymentMethod([expiredCard, validCard, validLink])
+
+      expect(result).toBeDefined()
+      expect(result?.id).toBe('pm_valid')
+    })
+
+    it('should return the first valid when multiple valid payment methods exist', () => {
+      const validCard1 = createCardPaymentMethod('pm_card_1', 2099, 12)
+      const validCard2 = createCardPaymentMethod('pm_card_2', 2050, 6)
+      const validLink = createLinkPaymentMethod('pm_link')
+
+      const result = findValidPaymentMethod([validCard1, validCard2, validLink])
+
+      expect(result).toBeDefined()
+      expect(result?.id).toBe('pm_card_1')
+    })
+
+    it('should return link if it appears before valid cards', () => {
+      const validLink = createLinkPaymentMethod('pm_link')
+      const validCard = createCardPaymentMethod('pm_card', 2099, 12)
+
+      const result = findValidPaymentMethod([validLink, validCard])
+
+      expect(result).toBeDefined()
+      expect(result?.id).toBe('pm_link')
+    })
+
+    it('should skip invalid methods at the start and return first valid', () => {
+      const expiredCard1 = createCardPaymentMethod('pm_expired_1', 2020, 1)
+      const expiredCard2 = createCardPaymentMethod('pm_expired_2', 2019, 6)
+      const sepa = createPaymentMethodWithType('pm_sepa', 'sepa_debit')
+      const validCard = createCardPaymentMethod('pm_valid', 2099, 12)
+      const validLink = createLinkPaymentMethod('pm_link')
+
+      const result = findValidPaymentMethod([
+        expiredCard1,
+        expiredCard2,
+        sepa,
+        validCard,
+        validLink,
+      ])
+
+      expect(result).toBeDefined()
+      expect(result?.id).toBe('pm_valid')
+    })
+
+    it('should return the only valid payment method even if last in list', () => {
+      const expiredCard1 = createCardPaymentMethod('pm_expired_1', 2020, 1)
+      const expiredCard2 = createCardPaymentMethod('pm_expired_2', 2019, 6)
+      const sepa = createPaymentMethodWithType('pm_sepa', 'sepa_debit')
+      const validLink = createLinkPaymentMethod('pm_link')
+
+      const result = findValidPaymentMethod([
+        expiredCard1,
+        expiredCard2,
+        sepa,
+        validLink,
+      ])
+
+      expect(result).toBeDefined()
+      expect(result?.id).toBe('pm_link')
+    })
+
+    it('should not mutate the original array', () => {
+      const validCard = createCardPaymentMethod('pm_valid', 2099, 12)
+      const expiredCard = createCardPaymentMethod('pm_expired', 2020, 1)
+      const original = [expiredCard, validCard]
+      const originalLength = original.length
+
+      findValidPaymentMethod(original)
+
+      expect(original).toHaveLength(originalLength)
+      expect(original[0].id).toBe('pm_expired')
+      expect(original[1].id).toBe('pm_valid')
+    })
+  })
 })