fix(mcp): respond -32601 to unknown request methods instead of silence

handle_request() returned None for unrecognized methods, and main() only
prints when a response exists - so unknown JSON-RPC requests got no reply
at all. Newer MCP clients probe servers before initializing: Google
Antigravity CLI (MCP protocol 2026-07-28) opens with a server/discover
request, and when leann_mcp stays silent it waits indefinitely - the
server shows "initializing..." forever in agy's MCP panel. Claude Code
and Gemini CLI never send the probe, which is why this was invisible
there.

Per JSON-RPC 2.0: an unknown request (with an id) now gets a -32601
Method-not-found error so clients can fall back; unknown notifications
(no id) still correctly get no reply.

Verified against Antigravity CLI 1.1.3's captured opening bytes:
server/discover gets its error, the client falls back to initialize,
and the server settles immediately with all tools listed. Claude Code
behavior unchanged.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: jakassebaum

packages/leann-core/src/leann/mcp.py

@@ -362,6 +362,17 @@ def handle_request(request):
         except Exception as e:
             return _make_error(request_id, str(e))
 
+    # Unknown method: a request (with an id) must get a -32601 error response
+    # so strict clients can fall back — e.g. Google Antigravity CLI probes
+    # with a server/discover request before initialize and hangs forever
+    # ("initializing...") if the server stays silent. Notifications (no id)
+    # must get no reply at all (JSON-RPC 2.0).
+    if request_id is not None:
+        return {
+            "jsonrpc": "2.0",
+            "id": request_id,
+            "error": {"code": -32601, "message": "Method not found"},
+        }
     return None