test: add ASP.NET Core blocking detection integration tests

[z0rimo]

Summary

Adds ASP.NET Core integration coverage for CaptureBlockingCalls.

The tests verify that:

• multiple requests create transient SentryMiddleware instances while reusing the same IBlockingMonitor and TaskBlockingListener
• a synchronous blocking call is captured as a Sentry event with the BlockingCallDetector mechanism
• disabling CaptureBlockingCalls avoids creating the blocking detector services and does not capture a blocking-call event

This uses the existing TestServer integration test infrastructure and does not change production code.

Fixes #5394

Testing

NO_MOBILE=true dotnet test test/Sentry.AspNetCore.Tests/Sentry.AspNetCore.Tests.csproj -f net10.0

Passed: 289
Failed: 0
Skipped: 0

The three new tests were also run 10 consecutive times with no failures.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 77.22%. Comparing base (3985304) to head (2693b92).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5578      +/-   ##
==========================================
+ Coverage   74.75%   77.22%   +2.47%     
==========================================
  Files         515      467      -48     
  Lines       18963    17229    -1734     
  Branches     3694     3385     -309     
==========================================
- Hits        14175    13305     -870     
+ Misses       3909     3157     -752     
+ Partials      879      767     -112     

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[jamescrosswell]

Thanks @z0rimo - overall, looks good. I made a few comments/suggestions.