Remove refresh_token from cookie instead of setting to NULL

[kailash-b]

Description

This PR improves the handling of refresh token failures in the Auth0 ASP.NET Core Authentication library.

Fix

• When a refresh token fails to refresh, the code now removes the refresh token from the authentication properties to ensure that the OnMissingRefreshToken event is triggered on subsequent requests.
• A new integration test has been added to verify that OnMissingRefreshToken is called after a refresh token failure and subsequent request.

References

• SDK-7171
• Failed refresh is silently ignored #191

Testing

• Should_Call_OnMissingRefreshToken_After_Refresh_Fails test added to simluate simulate a scenarios where refresh fails.

• This change adds test coverage for new/changed/fixed functionality

Checklist

• I have added documentation for new/changed functionality in this PR or in auth0.com/docs
• All active GitHub checks for tests, formatting, and security are passing
• The correct base branch is being used, if not main

Note : This fix is based on the issue raised by janruo in #191 . The fix will be attributed to @janruo during release.