fix: warn on positional load_session session_id

Author: PsiACE

src/acp/client/connection.py

@@ -1,6 +1,7 @@
 from __future__ import annotations
 
 import asyncio
+import warnings
 from collections.abc import Callable
 from typing import Any, cast, final
 
@@ -116,6 +117,12 @@ async def load_session(
     ) -> LoadSessionResponse:
         if session_id is _MISSING:
             if isinstance(mcp_servers, str):
+                warnings.warn(
+                    "Passing session_id as the second positional argument to load_session() is deprecated; "
+                    "use load_session(cwd=..., session_id=..., mcp_servers=...) instead.",
+                    DeprecationWarning,
+                    stacklevel=2,
+                )
                 session_id = mcp_servers
                 mcp_servers = None
             else:

tests/real_user/test_issue_55_mcp_servers_optional.py

@@ -62,6 +62,13 @@ async def test_session_requests_allow_missing_mcp_servers(server) -> None:
     )
     assert isinstance(load_session, LoadSessionResponse)
 
+    with pytest.warns(DeprecationWarning):
+        load_session = await asyncio.wait_for(
+            agent_conn.load_session("/workspace", new_session.session_id),
+            timeout=1.0,
+        )
+    assert isinstance(load_session, LoadSessionResponse)
+
     assert captured_agent, "Agent was not constructed"
     [agent] = captured_agent
     assert agent.seen_new_session == ("/workspace", None)