Add pytest cancellation subprocess regression coverage (microsoft#26058)

Pytest cancellation must terminate the child process in the
non-environment-extension execution path. This change adds regression
coverage for that behavior.

- **Subprocess tracking**
  - Retain the spawned process for cancellation handling.
- **Regression coverage**
  - Cancel an active pytest run.
  - Verify the child process receives `kill()`.

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: Eleanor Boyd <26030610+eleanorjboyd@users.noreply.github.com>

Author: Copilot

src/client/testing/testController/pytest/pytestExecutionAdapter.ts

@@ -246,6 +246,7 @@ export class PytestTestExecutionAdapter implements ITestExecutionAdapter {
                 });
 
                 const result = execService?.execObservable(runArgs, spawnOptions);
+                resultProc = result?.proc;
 
                 // Take all output from the subprocess and add it to the test output channel. This will be the pytest output.
                 // Displays output to user and ensure the subprocess doesn't run into buffer overflow.

src/test/testing/testController/pytest/pytestExecutionAdapter.unit.test.ts

@@ -2,7 +2,7 @@
 //  Copyright (c) Microsoft Corporation. All rights reserved.
 //  Licensed under the MIT License.
 import * as assert from 'assert';
-import { TestRun, Uri, TestRunProfileKind, DebugSessionOptions } from 'vscode';
+import { CancellationTokenSource, TestRun, Uri, TestRunProfileKind, DebugSessionOptions } from 'vscode';
 import * as typeMoq from 'typemoq';
 import * as sinon from 'sinon';
 import * as path from 'path';
@@ -182,6 +182,28 @@ suite('pytest test execution adapter', () => {
             typeMoq.Times.once(),
         );
     });
+    test('cancelling pytest execution kills the subprocess', async () => {
+        utilsWriteTestIdsFileStub.resolves('testIdPipe-mockName');
+        utilsStartRunResultNamedPipeStub.callsFake((_callback, deferredTillServerClose, token) => {
+            token?.onCancellationRequested(() => deferredTillServerClose.resolve());
+            return Promise.resolve('runResultPipe-mockName');
+        });
+        const cancellationToken = new CancellationTokenSource();
+        const testRun = typeMoq.Mock.ofType<TestRun>();
+        testRun.setup((t) => t.token).returns(() => cancellationToken.token);
+        const killStub = sinon.stub(mockProc, 'kill');
+        const uri = Uri.file(myTestPath);
+        adapter = new PytestTestExecutionAdapter(configService);
+
+        const execution = adapter.runTests(uri, [], TestRunProfileKind.Run, testRun.object, execFactory.object);
+        await deferred4.promise;
+        cancellationToken.cancel();
+
+        sinon.assert.calledOnce(killStub);
+        mockProc.emit('close', 0, null);
+        await execution;
+        cancellationToken.dispose();
+    });
     test('pytest execution respects settings.testing.cwd when present', async () => {
         const deferred2 = createDeferred();
         const deferred3 = createDeferred();