apply copilot comments

Author: Bertk

src/coverlet.core/Instrumentation/CecilAssemblyResolver.cs

@@ -304,7 +304,10 @@ public RuntimeConfigurationReader(string runtimeConfigFile)
       using var configuration = JsonDocument.Parse(jsonString, documentOptions);
 
       JsonElement rootElement = configuration.RootElement;
-      JsonElement runtimeOptionsElement = rootElement.GetProperty("runtimeOptions");
+      if (!rootElement.TryGetProperty("runtimeOptions", out JsonElement runtimeOptionsElement))
+      {
+        throw new InvalidOperationException($"The 'runtimeOptions' property is missing in the runtime configuration file {_runtimeConfigFile}.");
+      }
 
       if (runtimeOptionsElement.TryGetProperty("framework", out JsonElement frameworkElement))
       {

test/coverlet.integration.tests/DeterministicBuild.cs

@@ -49,7 +49,7 @@ private void CreateDeterministicTestPropsFile()
       _testProjectTfm = XElement.Load(Path.Combine(_testProjectPath, "coverlet.integration.determisticbuild.csproj"))!.
                        Descendants("PropertyGroup")!.Single().Element("TargetFramework")!.Value;
 
-      deterministicTestProps.Save(Path.Combine(propsFile));
+      deterministicTestProps.Save(propsFile);
     }
 
     private protected void AssertCoverage(string standardOutput = "", bool checkDeterministicReport = true)