Update dependencies to use httpx2 and refactor tests for compatibility

app/processing.py

@@ -4,7 +4,7 @@ import random
 from collections.abc import Awaitable, Callable
 from datetime import UTC, datetime
 
-import httpx
+import httpx2
 from sqlalchemy import select
 from sqlalchemy.ext.asyncio import async_sessionmaker
 
@@ -37,7 +37,7 @@ class WebhookClient:
         self._timeout = timeout
 
     async def send(self, url: str, payload: PaymentWebhook) -> None:
-        async with httpx.AsyncClient(timeout=self._timeout) as client:
+        async with httpx2.AsyncClient(timeout=self._timeout) as client:
             response = await client.post(url, json=payload.model_dump(mode="json"))
             response.raise_for_status()