fix: add type hints to session key helpers, extend context-local key to terminal_tool

- Add contextvars.Token[str] type hints to set/reset_current_session_key
- Use get_current_session_key(default='') in terminal_tool.py for background
  process session tracking, fixing the same env var race for concurrent
  gateway sessions spawning background processes

tools/approval.py

@@ -29,12 +29,12 @@ _approval_session_key: contextvars.ContextVar[str] = contextvars.ContextVar(
 )
 
 
-def set_current_session_key(session_key: str):
+def set_current_session_key(session_key: str) -> contextvars.Token[str]:
     """Bind the active approval session key to the current context."""
     return _approval_session_key.set(session_key or "")
 
 
-def reset_current_session_key(token) -> None:
+def reset_current_session_key(token: contextvars.Token[str]) -> None:
     """Restore the prior approval session key context."""
     _approval_session_key.reset(token)
 

tools/terminal_tool.py

@@ -1088,9 +1088,10 @@ def terminal_tool(
             # Spawn a tracked background process via the process registry.
             # For local backends: uses subprocess.Popen with output buffering.
             # For non-local backends: runs inside the sandbox via env.execute().
+            from tools.approval import get_current_session_key
             from tools.process_registry import process_registry
 
-            session_key = os.getenv("HERMES_SESSION_KEY", "")
+            session_key = get_current_session_key(default="")
             effective_cwd = workdir or cwd
             try:
                 if env_type == "local":