Fix Duo Chat staging evaluation is timing out

What does this MR do and why?

This MR fixes that staging evaluation of Duo Chat is failing due to the rack timeout error. We're currently performing a message serialization on all message rows of a thread, which is unnecessary because we only populate last 50 messages to LLM.

This is currently not an issue on production due to the rate limiter on GraphQL API. The staging eval uses a different and internal endpoint.

See https://gitlab.slack.com/archives/C05CJ1T3P0W/p1738649916648039 for more information.

Closes Duo Chat is getting slower as the message count... (#517058 - closed)

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