fix(Playwright): Guard _init() to prevent duplicate selector registration

- Only call _init() once per process using defaultSelectorEnginesInitialized flag
- Prevents duplicate selector registration errors in worker processes
- Add error handling for newContext when selectors already registered

Note: Worker processes still have initialization issues (ES Module loader errors)
that are unrelated to selector registration. The BROWSER_RESTART=session with
workers shows selector already registered errors which need further investigation.

Author: kobenguyent

lib/helper/Playwright.js

@@ -1180,7 +1180,10 @@ class Playwright extends Helper {
 
   async _startBrowser() {
     // Ensure custom locator strategies are registered before browser launch
-    await this._init()
+    // Only init once globally to avoid selector re-registration in workers
+    if (!defaultSelectorEnginesInitialized) {
+      await this._init()
+    }
 
     if (this.isElectron) {
       this.browser = await playwright._electron.launch(this.playwrightOptions)