feat(test): attach accessibility tree on test failure via saveTreeOnFailure

[farhanlabib]

Motivation

When a test fails, the screenshot tells you what the screen looks like, but not why the locator didn't match. The accessibility tree has that answer — element labels, types, visibility flags, and bounds — but there's currently no way to capture it automatically on failure.

What changed

• Added saveTreeOnFailure?: boolean to MobilewrightConfig in packages/mobilewright/src/config.ts
• Added a saveTreeOnFailure fixture option in @mobilewright/test (same pattern as autoAppLaunch) that reads the value from config, defaulting to false
• When a test fails and saveTreeOnFailure: true, the screen fixture calls viewTree() and attaches the result as view-tree-on-failure (application/json) to the test report — visible alongside the screenshot in the HTML report

The feature is opt-in and off by default, so existing projects are unaffected.

Usage

// mobilewright.config.js
module.exports = defineConfig({
  saveTreeOnFailure: true,
});

The view-tree-on-failure attachment appears in the HTML report under the failed test, containing the full ViewNode[] tree serialised as indented JSON.

Test plan

• Run a passing test with saveTreeOnFailure: true — no attachment added
• Run a failing test with saveTreeOnFailure: true — view-tree-on-failure JSON attachment appears in the HTML report alongside the screenshot
• Run a failing test with saveTreeOnFailure: false (default) — no JSON attachment, existing screenshot-on-failure still works normally
• Device disconnect mid-test — failure is reported cleanly, no crash from the viewTree call (covered by the inner try/catch)