Chromium proxy authentication with webRequest.onAuthRequired extension not working (MV3)

I’m trying to open Chromium in headfull mode on my Linux machine using a custom extension to handle authenticated proxy servers. Despite configuring the webRequest.onAuthRequired listener in my background script, Chromium isn’t authenticating with the proxy.

Chromium Details:
Binaries downloaded from: https://commondatastorage.googleapis.com/chromium-browser-snapshots/index.html?prefix=Linux_x64/
Version: chrome.137.0.7151.55

Command to open Chromium:

timeout -k 10 1800 /path_to_chrome_binary/chrome --disable-popup-blocking --allow-running-insecure-content --ignore-certificate-errors --no-sandbox --remote-allow-origins=* --user-agent="Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/125.0.6422.112 Safari/537.36" --proxy-server='{proxy_host}:{proxy_port}' --user-data-dir={user_data_path} --remote-debugging-port=9732 --load-extension={path_to_extension}

(Note: {proxy_host}, {proxy_port}, {user_data_path}, {path_to_extension} are placeholders.)

Extension Details:
Since my proxy server is authenticated, I’ve created a custom extension with the following files:

manifest.json:

`{
  "manifest_version": 3,
  "name": "Proxy Authenticator",
  "version": "1.0",
  "description": "Handles proxy authentication",
  "permissions": [
    "proxy",
    "storage",
    "webRequest",
    "webRequestAuthProvider",
    "webRequestBlocking",
    "scripting"
  ],
  "host_permissions": ["<all_urls>"],
  "background": {
    "service_worker": "background.js"
  },
  "action": {
    "default_title": "Proxy Authenticator"
  }
}`

background.js:

`chrome.webRequest.onAuthRequired.addListener(
    function(details, callback) {
       console.log(`Auth challenge for: ${details.uri}`);
       callback({
          authCredentials: {
             username: '{proxy_username}', 
             password: '{proxy_password}'
          }
       });
    },
    { urls: ["<all_urls>"] },
    ['asyncBlocking']
);`

(Note: {proxy_username} and {proxy_password} are placeholders.)

What I’m experiencing:
Chromium launches, but it fails to connect through the authenticated proxy. The console.log inside onAuthRequired doesn’t seem to be triggered, or the authentication is not being applied correctly.

What am i doing wrong here?

Ensured the extension path in the Chromium command is correct.
Verified the proxy host and port are accurate.
Checked the proxy username and password are correct.
Confirmed manifest_version is 3 and background uses service_worker.