docs(amazonq): Documentation for setting up and running new UI E2E Tests w/ vscode-extension-tester (#7750)

## Problem
The AWS Toolkit repository lacks documentation for the UI testing
commands (test:ui:prepare, test:ui:install, test:ui:run, test:ui) and
the new e2e_new testing suite. Contributors need clear guidance on how
to run UI tests and understand the testing framework structure.

## Solution
Added comprehensive UI testing documentation covering:

- Command usage and workflow for the complete UI test suite

- Directory structure and organization of the e2e_new testing framework

- Guidelines for writing new UI tests with example code

- Troubleshooting common issues and debug techniques

- Prerequisites and setup requirements

---

- Treat all work as PUBLIC. Private `feature/x` branches will not be
squash-merged at release time.
- Your code changes must meet the guidelines in
[CONTRIBUTING.md](https://github.com/aws/aws-toolkit-vscode/blob/master/CONTRIBUTING.md#guidelines).
- License: I confirm that my contribution is made under the terms of the
Apache 2.0 license.

Author: surajrdy-aws

docs/TESTPLAN.md

@@ -42,6 +42,10 @@ The test suite has the following categories of tests:
     -   Live in `src/testE2E`
     -   These tests are heavier than Integration tests.
     -   Some E2E tests have a more complicated architecture, described in [TEST_E2E](./TEST_E2E.md)
+-   UI E2E Tests: **slow** tests
+    -   Live in `packages/amazonq/test/e2e_new`
+    -   These tests target the UI testing aspect in an E2E sense.
+    -   Some E2E tests should be set up using pre-defined helpers and architecture, described in [UI_E2E_Test](./TUI_E2E_Test.md)
 -   Performance Tests: **slow** tests
     -   Live in `src/testInteg/perf`.
     -   A subset of integration tests focused on catching performance regressions.

docs/UI_E2E_Test.md

@@ -0,0 +1,125 @@
+## UI Testing
+
+UI tests use [vscode-extension-tester](https://github.com/redhat-developer/vscode-extension-tester) to test the Amazon Q extension in a real VS Code environment.
+
+### Quick Start
+
+```bash
+# Run complete UI test suite
+npm run test:ui
+```
+
+Note: All of these commands must be run at the root level aws-toolkit-vscode directory.
+
+### Individual Commands
+
+#### `test:ui:prepare`
+
+Downloads VS Code and ChromeDriver to `~/.vscode-test-resources` in order to properly hook Selenium/VET to our new VSCode instance we launch. Without this, test cannot be run at all.
+
+```bash
+npm run test:ui:prepare
+```
+
+#### `test:ui:install`
+
+Packages the Amazon Q extension VSIX and installs it for testing. This reflects local changes within your immediate aws-toolkit-vscode directory.
+
+```bash
+npm run test:ui:install
+```
+
+-   Runs `npm run package` in amazonq directory
+-   Extracts version from build output
+-   Installs VSIX using `extest install-vsix`
+-   Sets up extension in test environment
+
+#### `test:ui:run`
+
+Compiles TypeScript and runs UI tests within the dist directory. You must run this everytime you make a new change to your tests in order to recompile the tests.
+
+```bash
+npm run test:ui:run
+```
+
+-   Compiles test files with `npm run testCompile`
+-   Runs tests matching `packages/amazonq/dist/test/e2e_new/amazonq/tests/*.js`
+
+#### Authentication
+
+Currently, authentication is not configured to be automatically logged into AmazonQ. To bypass this for now (as of July 24th, 2025), you must click the approve/open button that redirects you to a browser in order for tests to be run in an authenticated environment at the start of a new.
+
+```bash
+npm run test:ui:run
+```
+
+#### Test Categories
+
+-   **Chat** - Amazon Q chat functionality
+-   **Pin Context** - Context pinning features
+-   **Quick Actions** - Quick action commands
+-   **Switch Model** - Model switching functionality
+
+### Writing New Tests
+
+1. Create test files in `packages/amazonq/test/e2e_new/amazonq/tests/`
+2. Import utilities from `../utils/`
+3. Use helpers from `../helpers/`
+4. Follow existing patterns for setup/cleanup
+
+#### Writing New Helpers
+
+1. Plan out abstractions/helpers after confirming current helpers are not viable
+2. Create helper file in `../helpers/`
+3. Following existing pattens for helper conventions and parameters
+
+#### Example Test Structure
+
+```typescript
+import { describe, it, before, after } from 'mocha'
+import { setupTestContext, cleanupTestContext } from '../utils/setup'
+
+describe('Feature Tests', () => {
+    before(async () => {
+        await setupTestContext()
+    })
+
+    after(async () => {
+        await cleanupTestContext()
+    })
+
+    it('should test functionality', async () => {
+        // Test implementation
+    })
+})
+```
+
+### Prerequisites
+
+-   Node.js and npm
+-   VS Code extension development environment
+-   Chrome/Chromium browser
+
+### Troubleshooting
+
+#### Common Issues
+
+-   **VS Code download fails**: Check internet connection, retry `test:ui:prepare`
+-   **Extension install fails**: Ensure packaging succeeds
+-   **Tests won't start**: Verify ChromeDriver/Chrome compatibility
+-   **Permission errors**: Check `~/.vscode-test-resources` permissions
+
+#### Reset
+
+```bash
+# Reset test environment
+rm -rf ~/.vscode-test-resources
+npm run test:ui:prepare
+```
+
+### Test Development Tips
+
+-   Tests should be independent and run in any order
+-   Use existing utilities for common operations
+-   Clean up resources in `after` hooks
+-   Follow naming conventions from existing tests