Address path composition review feedback

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: stephentoub

dotnet/test/E2E/RpcExtensionsLoadedE2ETests.cs

@@ -269,10 +269,10 @@ public async Task Failed_Extension_Reports_Failed_Status()
         // The bootstrap will fork the child, the import will throw, the child
         // exits with code 1, and the runtime should mark it as "failed".
         var extName = $"crashing-ext-{Guid.NewGuid():N}";
-        var extDir = Path.Combine(Ctx.HomeDir, "extensions", extName);
+        var extDir = Path.Join(Ctx.HomeDir, "extensions", extName);
         Directory.CreateDirectory(extDir);
         File.WriteAllText(
-            Path.Combine(extDir, "extension.mjs"),
+            Path.Join(extDir, "extension.mjs"),
             "throw new Error('intentional startup failure');");
 
         var extId = $"user:{extName}";

dotnet/test/E2E/SessionMcpAndAgentConfigE2ETests.cs

@@ -324,7 +324,7 @@ await File.WriteAllTextAsync(
                 Command = "node",
                 Args =
                 [
-                    Path.Combine(testHarnessDir, "test-mcp-elicitation-server.mjs"),
+                    Path.Join(testHarnessDir, "test-mcp-elicitation-server.mjs"),
                     "--config",
                     configPath
                 ],