[app] Use AzureChatOpenAI.with_structured_output instead of PydanticOutputParser

### StatGPT Backend version

0.2.0

### What is the problem this feature will solve?

Currently, we use `PydanticOutputParser` to generate format instructions and populate them into the prompt template for parsing outputs into Pydantic models. `AzureChatOpenAI` now has native support of Pydantic models parsing through `with_structured_output` method

### What is the feature you are proposing to solve the problem?

Use `AzureChatOpenAI.with_structured_output`

### What alternatives have you considered?

_No response_