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.