Fix Windows teardown flake in ClientE2ETests

Remove 'using var' from three ClientE2ETests that also call ForceStopAsync in their finally block. The double-disposal (using Dispose → DisposeAsync → ForceStopAsync, plus the explicit ForceStopAsync) races on Windows when the CLI process/pipes are mid-teardown, causing OperationCanceledException to bubble up and fail an otherwise-passing test.

Matches the existing pattern in SessionFsE2ETests where ForceStopAsync is wrapped in try-catch to swallow teardown-only exceptions.

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

Author: MackinnonBuck

dotnet/test/E2E/ClientE2ETests.cs

@@ -68,7 +68,7 @@ public async Task Should_Force_Stop_Without_Cleanup()
     [Fact]
     public async Task Should_Get_Status_With_Version_And_Protocol_Info()
     {
-        using var client = new CopilotClient(new CopilotClientOptions { UseStdio = true });
+        var client = new CopilotClient(new CopilotClientOptions { UseStdio = true });
 
         try
         {
@@ -83,14 +83,16 @@ public async Task Should_Get_Status_With_Version_And_Protocol_Info()
         }
         finally
         {
-            await client.ForceStopAsync();
+            try { await client.ForceStopAsync(); }
+            catch (OperationCanceledException) { }
+            catch (IOException) { }
         }
     }
 
     [Fact]
     public async Task Should_Get_Auth_Status()
     {
-        using var client = new CopilotClient(new CopilotClientOptions { UseStdio = true });
+        var client = new CopilotClient(new CopilotClientOptions { UseStdio = true });
 
         try
         {
@@ -108,14 +110,16 @@ public async Task Should_Get_Auth_Status()
         }
         finally
         {
-            await client.ForceStopAsync();
+            try { await client.ForceStopAsync(); }
+            catch (OperationCanceledException) { }
+            catch (IOException) { }
         }
     }
 
     [Fact]
     public async Task Should_List_Models_When_Authenticated()
     {
-        using var client = new CopilotClient(new CopilotClientOptions { UseStdio = true });
+        var client = new CopilotClient(new CopilotClientOptions { UseStdio = true });
 
         try
         {
@@ -144,7 +148,9 @@ public async Task Should_List_Models_When_Authenticated()
         }
         finally
         {
-            await client.ForceStopAsync();
+            try { await client.ForceStopAsync(); }
+            catch (OperationCanceledException) { }
+            catch (IOException) { }
         }
     }