Fix managed browser page retrieval when no pages (#1137)

This pull request addresses the issue of handling default context pages when none are open.  
- Introduces a conditional check to determine if a page exists in the context.  
- If no pages exist, a new page is created via await context.new_page().

crawl4ai/browser_manager.py

@@ -964,7 +964,10 @@ class BrowserManager:
             pages = context.pages
             page = next((p for p in pages if p.url == crawlerRunConfig.url), None)
             if not page:
-                page = context.pages[0] # await context.new_page()
+                if pages:
+                    page = context.pages[0]
+                else:
+                    page = await context.new_page()
         else:
             # Otherwise, check if we have an existing context for this config
             config_signature = self._make_config_signature(crawlerRunConfig)