Fix AttributeError in perplexity_async and update supported models (gpt 5.2 and grok 4.1)

[luizslvr]

Description

This PR addresses an AttributeError in the perplexity_async module and updates the list of supported models for Perplexity AI.

Changes Made

1. Added __init__.py to perplexity_async module:

   • Enables proper importing of Client, Emailnator, and LabsClient classes.
   • Mirrors the structure of the synchronous perplexity module.
   • Fixes AttributeError: module 'perplexity_async' has no attribute 'Client'.

2. Updated supported models:

   • Expanded the model lists in the search method assertions to include newly supported models like grok-4.1, gpt-5.2, etc., for various modes (pro, reasoning, etc.).

Why These Changes

• The missing __init__.py prevented users from importing the async client as shown in the documentation/examples.
• Model updates ensure compatibility with the latest Perplexity AI offerings.

Testing

• Verified that import perplexity_async; client = await perplexity_async.Client() now works without errors.
• Confirmed model assertions pass with the new lists.