Fix ValueError when no knowledge base is attached

_build_kb_mcp_server() returns a 3-tuple (server, tool_names,
tools_by_name) on its normal path, and all three call sites unpack
three values. But the early return for the "no KB attached" case still
returned a 2-tuple (None, []), so every message sent without a knowledge
base attached raised:

    ValueError: not enough values to unpack (expected 3, got 2)

Return (None, [], {}) instead, and update the docstring to match.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

claude_agent_pipe.py

@@ -369,8 +369,8 @@ def _build_kb_mcp_server(
     user_dict: Optional[Dict[str, Any]] = None,
     event_emitter: Optional[Callable] = None,
 ):
-    """Return (mcp_config, tool_names) for a knowledge-base search tool Claude
-    can invoke, or (None, []) if no KBs are attached.
+    """Return (mcp_config, tool_names, tools_by_name) for a knowledge-base
+    search tool Claude can invoke, or (None, [], {}) if no KBs are attached.
 
     Result formatting follows OpenWebUI's native RAG shape (<source id=N ...>
     tags + citation event), so Claude's replies render with inline [N]
@@ -379,7 +379,7 @@ def _build_kb_mcp_server(
     that OpenWebUI's middleware already filtered by the user's grants.
     """
     if not knowledge:
-        return None, []
+        return None, [], {}
 
     collection_names = [k["id"] for k in knowledge]
     display = ", ".join(k["name"] for k in knowledge)