Clarify OpenHands naming (replace “OSS” wording in docs and backend) (#12235)

Co-authored-by: openhands <openhands@all-hands.dev>

openhands/server/types.py

@@ -12,9 +12,12 @@ from typing import Any, ClassVar, Protocol
 
 
 class AppMode(Enum):
-    OSS = 'oss'
+    OPENHANDS = 'oss'
     SAAS = 'saas'
 
+    # Backwards-compatible alias (deprecated): prefer AppMode.OPENHANDS
+    OSS = 'oss'
+
 
 class SessionMiddlewareInterface(Protocol):
     """Protocol for session middleware classes."""