[nwjs/nw.js] a0fc40: test: add MV3 DevTools extension test case

[commits...@nwjs.io]

Branch: refs/heads/main
Home: https://api.github.com/repos/nwjs/nw.js
Commit: a0fc40a385e47ebc3b5f3e37f29dde90b849b262
https://github.com/nwjs/nw.js/commit/a0fc40a385e47ebc3b5f3e37f29dde90b849b262
Author: Kevin Fan <k...@gnor.net>
Date: 2026-05-17 16:31:07 -0600 -0600

Changed paths:
A test/sanity/mv3-devtools-extension/index.html
A test/sanity/mv3-devtools-extension/mv3-extension/devtools.html
A test/sanity/mv3-devtools-extension/mv3-extension/devtools.js
A test/sanity/mv3-devtools-extension/mv3-extension/icon.png
A test/sanity/mv3-devtools-extension/mv3-extension/manifest.json
A test/sanity/mv3-devtools-extension/mv3-extension/panel.html
A test/sanity/mv3-devtools-extension/package.json
A test/sanity/mv3-devtools-extension/test.py

Log Message:
-----------
test: add MV3 DevTools extension test case

Add a sanity test that verifies MV3 DevTools extensions load correctly
in nw.js. The test loads a minimal MV3 extension with a devtools_page,
opens DevTools, and checks that the extension's panel tab appears.

This covers the fix for the issue where MV3 DevTools extensions without
a background service worker fail to load subresources because the
browser-side resource policy only allowed devtools:// initiators,
not the extension's own origin.

Fixes #8353