AI-Assisted Unit Test Generation for Android

Here is CheckoutViewModel and its UiState. List the behaviours and edge cases worth testing:
user actions, success/failure paths, boundary values, concurrency (double taps), and
state after errors. Don't write code yet. Group them by priority.

Write JUnit4 tests for cases 1, 2, 4 and 7 only.
Conventions: MainDispatcherRule from com.example.testing, runTest, FakeCartRepository
and FakePaymentRepository (no mocking libraries), backtick test names describing behaviour,
one behaviour per test, assert on uiState values only.

class CheckoutViewModelTest {
    @get:Rule val mainRule = MainDispatcherRule()

    @Test
    fun `double tap on pay submits only one payment`() = runTest {
        val payments = FakePaymentRepository(delayMs = 1_000)
        val vm = CheckoutViewModel(FakeCartRepository(total = 49_900), payments)

        vm.onPayClick(); vm.onPayClick()
        advanceUntilIdle()

        assertEquals(1, payments.submittedCount)                      // guards against duplicate charges
    }

    @Test
    fun `payment failure keeps cart and shows retryable error`() = runTest {
        val vm = CheckoutViewModel(FakeCartRepository(total = 49_900), FakePaymentRepository(fail = true))

        vm.onPayClick(); advanceUntilIdle()

        val state = vm.uiState.value
        assertEquals(49_900, state.totalPaise)
        assertTrue(state.error?.canRetry == true)
        assertFalse(state.isProcessing)
    }
}