fix: ReferralMiddleware causing blank pages with rid query parameter

[Copilot]

Problem

Pages with rid query parameter (e.g., /guidelines?rid=abc123) were returning blank content with HTTP 200 status, while the same pages without the parameter worked correctly.

Root Cause

The ReferralMiddleware was processing referral tracking but failing to continue the request pipeline when a rid parameter was present. The middleware was missing the critical await _Next(context); call after referral processing, causing the request to terminate early.

Before (Broken):

public async Task InvokeAsync(HttpContext context, IReferralService referralService, UserManager<EssentialCSharpWebUser> userManager)
{
    string? referralId = query["rid"];
    if (string.IsNullOrWhiteSpace(referralId))
    {
        await _Next(context);  // ✓ Continues pipeline when NO rid
        return;
    }
    
    // Process referral tracking
    referralService.TrackReferralAsync(referralId, claimsUser);
    
    // ❌ MISSING: await _Next(context);
    // Pipeline stops here when rid exists!
}

After (Fixed):

public async Task InvokeAsync(HttpContext context, IReferralService referralService, UserManager<EssentialCSharpWebUser> userManager)
{
    string? referralId = query["rid"];
    if (string.IsNullOrWhiteSpace(referralId))
    {
        await _Next(context);
        return;
    }
    
    try
    {
        // Process referral tracking
        referralService.TrackReferralAsync(referralId, claimsUser);
    }
    catch (Exception ex)
    {
        _logger.LogError(ex, "Failed to track referral ID {ReferralId}", referralId);
    }
    
    // ✅ FIXED: Always continue the pipeline
    await _Next(context);
}

Changes Made

1. Critical Fix: Added missing await _Next(context); call after referral processing
2. Error Resilience: Added try-catch around referral tracking to prevent exceptions from breaking page rendering
3. Logging Enhancement: Added structured error logging for debugging referral tracking issues
4. Test Coverage: Added comprehensive tests to verify the fix works for all scenarios

Verification

✅ /guidelines?rid=abc123 now renders content correctly
✅ /about?rid=user-ref now renders content correctly
✅ Referral tracking functionality preserved
✅ Error scenarios handled gracefully
✅ Backward compatibility maintained

Test Cases Added

• Pages with valid rid parameters return content
• Pages with empty/whitespace rid parameters work correctly
• Pages with non-rid parameters continue to work
• Error scenarios are logged but don't break page rendering

Fixes #756.

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