Fix memory extraction json format issue

[TaoChenOSU]

Motivation and Context

Model hasn't been generating JSON consistently for memory extraction.

Description

Set the response format to JSON to force the model to output valid json for memory extraction.

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows the Contribution Guidelines and the pre-submission formatting script raises no violations
• All unit tests pass, and I have added new tests where possible
• I didn't break anyone 😄