Registers a GType for a tool configuration key (static method).
When deserializing JSON, Config2 will use the registered GType to deserialize the tool configuration using standard Json.Serializable. This is used for the tools map.
| tool_name |
The tool name (e.g., "codebase_search", "google_search") |
| gtype |
The GType to use for deserialization (must extend BaseToolConfig) |