Upgrade to xUnit v3

[askpt]

Requirements

Upgrade test framework dependencies to xUnit v3. Recent Reqnroll v3.1 release provides official compatibility with xUnit v3 (see: https://reqnroll.net/news/2025/09/reqnroll-v3-1-released/#:~:text=xUnit%20v3%20support%3A%20You%20can%20now%20use%20Reqnroll,scenario-level%20tags%20for%20NUnit%2C%20MsTest%20V2%2C%20and%20TUnit.).

Tasks:

• Update all xUnit dependencies to v3
• Validate Reqnroll integration with xUnit v3
• Update test adapters and CI configuration if necessary
• Ensure all tests run and pass after upgrade