Address code review feedback for War Thunder HTTP polling implementation

[Copilot]

Resolves 7 code review issues from the War Thunder realtime source PR including broken documentation links, missing using statements, incorrect cancellation handling, and resource disposal issues.

Changes

Cancellation Handling

• Distinguish between user-initiated cancellation and request timeout in HttpPollingSourceBase
• User cancellations exit cleanly; timeouts trigger retry logic with exponential backoff
• Remove redundant TaskCanceledException catch (already handled by OperationCanceledException parent)

catch (OperationCanceledException ex)
{
    if (cancellationToken.IsCancellationRequested)
    {
        // User requested cancellation - exit cleanly
        yield break;
    }
    
    // Timeout from linkedCts - treat as connection error and retry
    _consecutiveErrors++;
    // ... exponential backoff logic
}

Resource Management

• Add try-catch-finally block to CaptureWarThunderSession matching F1 pattern
• Properly dispose CancellationTokenSource and unregister event handler in finally block
• Catch FileNotFoundException and OperationCanceledException with user-friendly messages

Compilation Fixes

• Add missing System, System.Collections.Generic, and System.Linq using statements to HttpPollingSourceOptions

Documentation

• Replace broken links to non-existent API_REFERENCE.md and PERFORMANCE.md with "(coming soon)" placeholders

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