Add comprehensive test coverage for payment flows

[callebtc]

Description:
Critical payment flows lack comprehensive test coverage, creating risk for production issues in a POS application where payment reliability is essential.

Areas Needing Test Coverage:

1. Payment Flow Testing:

• Happy path: successful payment processing
• Error scenarios: network failures, invalid amounts
• NFC operation failures and recovery
• Payment method selection logic

2. Wallet Operations:

• CashuWalletManager initialization
• Multi-mint wallet operations
• Balance calculations and updates
• Database migration scenarios

3. Critical Edge Cases:

• Rapid NFC card tap/remove cycles
• App backgrounding during payment
• Low memory scenarios
• Network connectivity issues

Fix Instructions:

1. Add test dependencies:

dependencies {
    testImplementation "junit:junit:4.13.2"
    testImplementation "org.mockito:mockito-core:5.1.1"
    testImplementation "androidx.test:core:1.5.0"
    testImplementation "org.robolectric:robolectric:4.11.1"
    
    androidTestImplementation "androidx.test.ext:junit:1.1.5"
    androidTestImplementation "androidx.test.espresso:espresso-core:3.5.1"
}

2. Create test structure:

app/src/test/java/
├── com/electricdreams/numo/
│   ├── payment/PaymentFlowTest.kt
│   ├── cashu/CashuWalletManagerTest.kt  
│   ├── nfc/NfcOperationsTest.kt
│   └── ui/PosUiCoordinatorTest.kt

Priority Tests:

1. Payment processing reliability
2. Wallet balance calculations
3. NFC error handling
4. UI state management

Target: >80% coverage for critical payment paths