test(amazonq): implement performance test for downloadExportResultArchieve. (aws#5710)

## Problem
Continuation of work here:
aws#5670

## Solution
In this case `downloadExportResultArchieve` is reading a large
collection of objects into a buffer, then writing that buffer to a file.
We test this with varying collections of objects. We test 1x1KB,
10x100B, 100x10B, and 1000x1B.

---

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License: I confirm that my contribution is made under the terms of the
Apache 2.0 license.

Author: Hweinstock

packages/core/src/testInteg/perf/downloadExportResultArchive.test.ts

@@ -0,0 +1,98 @@
+/*!
+ * Copyright Amazon.com, Inc. or its affiliates. All Rights Reserved.
+ * SPDX-License-Identifier: Apache-2.0
+ */
+import assert from 'assert'
+import { WorkspaceFolder } from 'vscode'
+import { ExportResultArchiveCommandInput } from '@amzn/codewhisperer-streaming'
+import * as sinon from 'sinon'
+import path from 'path'
+import { fs, getRandomString } from '../../shared'
+import { createTestWorkspace } from '../../test/testUtil'
+import { performanceTest } from '../../shared/performance/performance'
+import { downloadExportResultArchive } from '../../shared/utilities/download'
+
+interface SetupResult {
+    workspace: WorkspaceFolder
+    exportCommandInput: ExportResultArchiveCommandInput
+    writeFileStub: sinon.SinonStub
+    cwStreaming: any
+}
+
+interface FakeCommandOutput {
+    body: { binaryPayloadEvent: { bytes: Buffer } }[]
+}
+
+function generateCommandOutput(numPieces: number, pieceSize: number): FakeCommandOutput {
+    const body = Array.from({ length: numPieces }, (_, i) => {
+        return {
+            binaryPayloadEvent: {
+                bytes: Buffer.from(getRandomString(pieceSize)),
+            },
+        }
+    })
+    return { body }
+}
+
+async function setup(pieces: number, pieceSize: number): Promise<SetupResult> {
+    // Force VSCode to find test workspace only to keep test contained and controlled.
+    const workspace = await createTestWorkspace(1, {})
+    const exportCommandInput = {} as ExportResultArchiveCommandInput
+    // Manutally stub the CodeWhispererStreaming to avoid constructor call.
+    const cwStreaming = { exportResultArchive: () => generateCommandOutput(pieces, pieceSize) }
+
+    const writeFileStub = sinon.stub(fs, 'writeFile')
+    return { workspace, exportCommandInput, writeFileStub, cwStreaming }
+}
+
+function perfTest(pieces: number, pieceSize: number, label: string) {
+    return performanceTest(
+        {
+            testRuns: 10,
+            linux: {
+                userCpuUsage: 200,
+                systemCpuUsage: 35,
+                heapTotal: 4,
+            },
+            darwin: {
+                userCpuUsage: 200,
+                systemCpuUsage: 35,
+                heapTotal: 4,
+            },
+            win32: {
+                userCpuUsage: 200,
+                systemCpuUsage: 35,
+                heapTotal: 4,
+            },
+        },
+        label,
+        function () {
+            return {
+                setup: async () => await setup(pieces, pieceSize),
+                execute: async ({ workspace, cwStreaming, exportCommandInput, writeFileStub }: SetupResult) => {
+                    await downloadExportResultArchive(
+                        cwStreaming,
+                        exportCommandInput,
+                        path.join(workspace.uri.fsPath, 'result')
+                    )
+                },
+                verify: async (setup: SetupResult) => {
+                    assert.ok(setup.writeFileStub.calledOnce)
+                    assert.ok((setup.writeFileStub.firstCall.args[1] as Buffer).length === pieces * pieceSize)
+                },
+            }
+        }
+    )
+}
+
+describe('downloadExportResultArchive', function () {
+    describe('performanceTests', function () {
+        afterEach(function () {
+            sinon.restore()
+        })
+        perfTest(1, 1000, '1x1KB')
+        perfTest(10, 100, '10x100B')
+        perfTest(100, 10, '100x10B')
+        perfTest(1000, 1, '1000x1B')
+    })
+})