Add comprehensive Invoice API test suite

[devin-ai-integration]

Add comprehensive Invoice API test suite

Summary

Created a comprehensive test suite for all Invoice API endpoints following the existing testing patterns from test_jwt.py. The new test_invoice_api.py file includes 21 test cases covering all CRUD operations (GET, POST, PUT, DELETE) for the /api/invoices endpoints with comprehensive status code testing (200/201, 400, 401, 404, 500).

Key Features:

• Complete CRUD operation coverage for Invoice API endpoints
• JWT authentication testing for authorized/unauthorized access scenarios
• Comprehensive validation testing for required fields and data formats
• User isolation testing (users can only access their own invoices)
• Invoice totals calculation verification
• Follows existing test patterns with proper setUp/tearDown methods
• Uses in-memory SQLite database for isolated testing
• Includes run_invoice_tests() function with proper exit codes

Test Results: All 28 tests pass (21 new Invoice API tests + 7 existing JWT tests)

Review & Testing Checklist for Human

• Verify test data matches real business requirements - Review the sample invoice data in get_sample_invoice_data() to ensure it reflects realistic invoice scenarios and matches your API's validation requirements
• Run tests locally - Execute python3 src/backend/tests/test_invoice_api.py and python3 src/backend/tests/run_tests.py to verify compatibility with your environment
• Validate endpoint coverage completeness - Confirm that all critical Invoice API endpoints and edge cases important to your business logic are covered
• Check authentication implementation - Verify that the JWT authentication setup in tests matches your production authentication patterns
• Test invoice calculation logic - Review the test_invoice_totals_calculation test to ensure it matches your expected tax and total calculation business rules

---

Diagram

%%{ init : { "theme" : "default" }}%%
graph TB
    subgraph "Test Infrastructure"
        TestRunner["src/backend/tests/<br/>run_tests.py"]:::context
        JWTTests["src/backend/tests/<br/>test_jwt.py"]:::context
        NewTests["src/backend/tests/<br/>test_invoice_api.py"]:::major-edit
    end
    
    subgraph "Application Code"
        InvoiceRoutes["src/backend/routes/<br/>invoices.py"]:::context
        Models["src/backend/<br/>models.py"]:::context
        App["src/backend/<br/>app.py"]:::context
    end
    
    subgraph "Testing Components"
        Auth["JWT Authentication<br/>Testing"]:::context
        CRUD["CRUD Operations<br/>Testing"]:::major-edit
        Validation["Input Validation<br/>Testing"]:::major-edit
    end

    TestRunner --> JWTTests
    TestRunner --> NewTests
    NewTests --> Auth
    NewTests --> CRUD
    NewTests --> Validation
    NewTests -.->|"Tests endpoints in"| InvoiceRoutes
    NewTests -.->|"Uses models from"| Models
    NewTests -.->|"Creates app from"| App
    JWTTests -.->|"Pattern followed"| NewTests

    subgraph Legend
        L1[Major Edit]:::major-edit
        L2[Minor Edit]:::minor-edit  
        L3[Context/No Edit]:::context
    end

    classDef major-edit fill:#90EE90
    classDef minor-edit fill:#87CEEB
    classDef context fill:#FFFFFF

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Notes

• The test suite follows the exact same patterns as the existing test_jwt.py file for consistency
• All tests use in-memory SQLite databases for isolation and speed
• The test data includes realistic invoice scenarios with multiple line items and tax calculations
• User isolation is properly tested to ensure data security between different users
• Error handling is comprehensively tested for all expected failure scenarios

Link to Devin run: https://app.devin.ai/sessions/473921fb2339474f89bc2b50492a1649
Requested by: @JRWu (jia.wu@codeium.com)

[devin-ai-integration]

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