Add async client support

- Add AsyncFastGPTClient, AsyncChatClient, AsyncAppClient classes
- Implement async/await patterns with httpx.AsyncClient
- Add async context manager support (async with, async for)
- Add async retry logic with exponential backoff
- Add 29 async unit tests covering all async client methods
- Update __init__.py to export async clients

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

fastgpt_client/__init__.py

@@ -3,6 +3,11 @@
 A Python client library for interacting with FastGPT's OpenAPI.
 """
 
+from fastgpt_client.async_client import (
+    AsyncAppClient,
+    AsyncChatClient,
+    AsyncFastGPTClient,
+)
 from fastgpt_client.client import AppClient, ChatClient, FastGPTClient
 from fastgpt_client.exceptions import (
     APIError,
@@ -18,6 +23,10 @@ __all__ = [
     "FastGPTClient",
     "ChatClient",
     "AppClient",
+    # Asynchronous clients
+    "AsyncFastGPTClient",
+    "AsyncChatClient",
+    "AsyncAppClient",
     # Exceptions
     "FastGPTError",
     "APIError",