fix(langgraph): merge callbacks in ensure_config instead of overwriting

[ccurme (ccurme)]

ensure_config used last-write-wins for the callbacks key, which caused callbacks set via .with_config() to be silently dropped when a later config also contained callbacks. This affected astream_events (and therefore stream_mode="events", which injects its own internal callback handler), overwriting any user-provided callbacks on self.config.

Here we update ensure_config to merge callbacks using the same logic as in merge_configs.