Run a tool only on a connection of the connector that provides it

At run time a tool now refuses a connection whose connector does not list
the tool's type (a Telegram bot token never reaches an ntfy or MCP tool),
and an MCP tool refuses a connection when no server is known for it,
instead of sending the key without checking. Legacy MCP rows still find
their server from the stored provider value.

docsgpt/connectors/catalog.py

@@ -479,7 +479,9 @@ def connector_key_for_row(row: dict) -> Optional[str]:
     """
     key = row.get("connector_key")
     provider = row.get("provider") or ""
-    if not key:
+    if not key or key.startswith("mcp:"):
+        # ``mcp:<server>`` is a provider value, never a catalog key.
+        key = None
         if provider.startswith("mcp:"):
             key = "custom_mcp"
         elif get_definition(provider):