Remove coverage file from tracking, add to gitignore

Agent-Logs-Url: https://github.com/zzzprojects/System.Linq.Dynamic.Core/sessions/82ed2f4e-0c76-407c-a330-d4043e96162f

Co-authored-by: StefH <249938+StefH@users.noreply.github.com>

Author: Copilot

.gitignore

@@ -238,4 +238,5 @@ _Pvt_Extensions
 /coverage.xml
 /dynamic-coverage-*.xml
 /test/**/coverage.net8.0.opencover.xml
+/test/**/coverage.opencover.xml
 .nuget/