[Reasoning Engine] LangchainAgent.stream_query attempts to import from deprecated langchain.load path

#### Environment details

- OS type and version: Ubuntu 24.04.4 LTS
- Python version: `3.12.10`
- pip version: `25.2`
- `google-cloud-aiplatform` version: `Latest from main`

#### Steps to reproduce

1. Initialize a `LangchainAgent` or `LanggraphAgent` from the agent engine or reasoning engine templates.
2. Call the `query` or `stream_query` method.
3. Observe that it fails with an `ImportError` because it attempts to import from `langchain.load` instead of `langchain_core.load`.

#### Code example

```python
from vertexai.agent_engines.templates.langchain import LangchainAgent

# Initialize agent
agent = LangchainAgent(model="gemini-1.5-flash-001")
agent.set_up()

# Both query and stream_query are affected
result = agent.query(input="Hello")

for chunk in agent.stream_query(input="Hello"):
print(chunk)
```

#### Stack trace
```
Traceback (most recent call last):
File "example.py", line 8, in
for chunk in agent.stream_query(input="Hello"):
File ".../vertexai/agent_engines/templates/langchain.py", line 665, in stream_query
from langchain.load import dump as langchain_load_dump
ImportError: cannot import name 'dump' from 'langchain.load'
```

#### Affected files

The same deprecated import (`from langchain.load import dump`) exists in both `query()` and `stream_query()` across all 4 template modules:

- `vertexai/agent_engines/templates/langchain.py`
- `vertexai/agent_engines/templates/langgraph.py`
- `vertexai/preview/reasoning_engines/templates/langchain.py`
- `vertexai/preview/reasoning_engines/templates/langgraph.py`

Making sure to follow these steps will guarantee the quickest resolution possible.

Thanks!

Inizia con query() e stream_query() nei quattro file di template interessati: vertexai/agent_engines/templates/langchain.py, langgraph.py e i template corrispondenti di preview/reasoning_engines. Confronta gli import ed esegui i test dei template oppure riproduci le chiamate a LangchainAgent e LanggraphAgent; il lavoro è completato quando entrambi i metodi non falliscono più a causa dell’import obsoleto langchain.load.

Scritto dal modello di indicizzazione a partire dal testo della issue.