Replace httpx and httpx-sse with httpx2 (#2972)

Co-authored-by: Max Isbey <224885523+maxisbey@users.noreply.github.com>

tests/client/test_http_unicode.py

@@ -7,7 +7,7 @@ Verifies that Unicode text is correctly transmitted and received in both directi
 from collections.abc import AsyncIterator
 from contextlib import asynccontextmanager
 
-import httpx
+import httpx2
 import mcp_types as types
 import pytest
 from mcp_types import TextContent, Tool
@@ -114,7 +114,7 @@ async def unicode_session() -> AsyncIterator[ClientSession]:
         session_manager.run(),
         # follow_redirects matches the SDK's own client factory; Starlette's Mount 307-redirects
         # the bare /mcp path to /mcp/.
-        httpx.AsyncClient(
+        httpx2.AsyncClient(
             transport=StreamingASGITransport(app), base_url=BASE_URL, follow_redirects=True
         ) as http_client,
         streamable_http_client(f"{BASE_URL}/mcp", http_client=http_client) as (read_stream, write_stream),