Fix blocking async calls to improve performance

[Blackbaud-PaulGibson]

Add CancellationToken support to async methods and revert to .NET 8.0

Summary

This PR addresses Paul Gibson's feedback on the original blocking async calls fix by adding comprehensive CancellationToken support to all async methods and reverting the project framework from .NET 6.0 back to .NET 8.0.

Key Changes:

• Added CancellationToken parameters to all async methods in IAuthenticationService, IConstituentsService, and IDataStorageService interfaces
• Updated all corresponding service implementations to accept and properly pass CancellationToken to HTTP operations
• Modified HTTP client calls (PostAsync, GetAsync, ReadAsStringAsync) to receive the CancellationToken for proper request cancellation
• Reverted skyapi-headless-data-sync.csproj from <TargetFramework>net6.0</TargetFramework> back to <TargetFramework>net8.0</TargetFramework>
• Added missing using System.Threading; statements where needed
• Updated the entire async call chain to propagate CancellationToken from service interfaces down to HTTP operations

Performance & Reliability Benefits:

• Network requests can now be properly cancelled, preventing resource waste and improving responsiveness
• Better alignment with .NET async patterns and best practices
• Maintains the original async/await performance improvements while adding cancellation support

Review & Testing Checklist for Human

• End-to-end functionality testing - Test the application with real API credentials and refresh token to ensure data sync still works correctly with the new CancellationToken-enabled methods
• Framework compatibility verification - Confirm the application builds and runs properly with .NET 8.0 in your deployment environment (since this reverts the previous .NET 6.0 change)
• Async error handling verification - Verify that error handling in DataSyncService.Constituent.cs and other services still catches and handles exceptions properly with the new async method signatures
• Interface implementation completeness - Double-check that no other implementations of IAuthenticationService, IConstituentsService, or IDataStorageService exist in the codebase that would be broken by the signature changes
• Cancellation behavior testing - Test scenarios where network requests might be cancelled (network timeouts, application shutdown) to ensure CancellationToken is working as expected

Recommended Test Plan:

1. Build and run the application with .NET 8.0 SDK
2. Set up the application with valid API credentials per the README
3. Run initial sync with dotnet run --refreshtoken <token>
4. Let the application run for several sync cycles to verify continuous operation
5. Test error scenarios (invalid tokens, network issues) to ensure proper error handling with new async signatures

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Diagram

%%{ init : { "theme" : "default" }}%%
graph TD
    ProjectFile["skyapi-headless-data-sync.csproj<br/>Reverted to .NET 8.0"]:::minor-edit
    
    IAuthService["IAuthenticationService.cs<br/>Added CancellationToken param"]:::major-edit
    AuthService["AuthenticationService.cs<br/>Updated implementation"]:::major-edit
    
    IConstService["IConstituentsService.cs<br/>Added CancellationToken param"]:::major-edit
    ConstService["ConstituentsService.cs<br/>Updated HTTP calls"]:::major-edit
    
    IDataStorage["IDataStorageService.cs<br/>Added CancellationToken param"]:::major-edit
    DataStorage["DataStorageService.cs<br/>Updated ReadAsStringAsync"]:::major-edit
    
    SyncService["DataSyncService.Constituent.cs<br/>Added using Threading"]:::minor-edit
    
    SyncService --> IConstService
    IConstService --> ConstService
    ConstService --> IAuthService
    ConstService --> IDataStorage
    IAuthService --> AuthService
    IDataStorage --> DataStorage
    AuthService --> IDataStorage

    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

• Breaking interface changes: All async method signatures were modified, so this is a significant change that affects the entire async call chain
• Build verification: The project now builds successfully with .NET 8.0, resolving the framework compatibility concern
• CancellationToken propagation: Used CancellationToken.None as default in the data sync service to maintain existing behavior while enabling cancellation support
• HTTP client improvements: All network operations now properly support cancellation, which is especially important for long-running sync operations

⚠️ Important: While the code compiles successfully and follows proper async patterns, comprehensive end-to-end testing with actual API credentials is essential to verify the CancellationToken changes work correctly in practice.

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Link to Devin run: https://app.devin.ai/sessions/379858f90d0748a88633f6006b0d0930
Requested by: Paul Gibson (@Blackbaud-PaulGibson)

[Blackbaud-PaulGibson]

@devin Most of this PR looks good. However, those async methods should take a CancellationToken to be given to the network request.

[Blackbaud-PaulGibson]

@devin-ai-integration[bot] Most of this PR looks good. However, those async methods should take a CancellationToken to be given to the network request. Also, why did you downgrade the project to .Net 6? Can you review that and figure out how to not need that change - we really don't want to go backwards on versions. Thanks!