This PR adds comprehensive test coverage for the WealthsimpleDownloader class, following the established testing patterns used for Token and WealthsimpleAccount classes.

What's Added

New Test File: Tests/WealthsimpleTests/WealthsimpleDownloaderTests.swift with 16 comprehensive test cases covering all public methods and major code paths:

Test Coverage

• Constructor (1 test): Validates proper instantiation
• Authentication (5 tests):
  • Token refresh scenarios (success/failure)
  • Credential storage handling (valid/empty)
  • New token authentication flows
• getAccounts (4 tests): No token, success, HTTP errors, network errors
• getPositions (3 tests): No token, success, network errors
• getTransactions (3 tests): No token, success, network errors

Infrastructure Enhancements

• Extended MockURLProtocol to support /positions and /transactions endpoints
• Added MockAccount helper struct implementing the Account protocol
• Proper JSON response mocking matching actual API formats

Implementation Details

The tests follow the existing patterns from TokenTests and WealthsimpleAccountTests:

• Use MockCredentialStorage for credential mocking
• Use MockURLProtocol for network request interception
• Use XCTestExpectation for async operation testing
• All network calls are mocked - no external dependencies

Example test structure:

func testGetAccountsWithTokenSuccess() throws {
    // Setup mock responses
    MockURLProtocol.accountsRequestHandler = { url, request in
        // Validate headers and return mock data
    }
    
    // Test the actual functionality
    downloader.getAccounts { result in
        // Verify expected behavior
    }
}

Quality Assurance

• ✅ All 56 tests pass (40 existing + 16 new)
• ✅ Builds without warnings using -Xswiftc -warnings-as-errors
• ✅ SwiftLint compliant with appropriate disable comments for file/function length only
• ✅ No external network calls in tests
• ✅ Comprehensive error scenario coverage

The tests ensure robust coverage of all error conditions, success paths, and edge cases while maintaining the existing code quality standards.

Fixes #262.

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