[dotnet] Avoid async void events in CDP Network session

[nvborisenko]

Description

async void is not good approach - unhandled exceptions will crash entire app.

Motivation and Context

Fixes #12480

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• I have read the contributing document.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.

[diemol]

Build is failing.

[titusfortner]

I'm so glad we fixed the .NET build finally 😂