[Due for payment 2026-08-28] [Due 2026-08-26] Migrate ProposalPolice to use responses+conversations rather than assistants

[roryabraham]

Problem

The assistants API we use for ProposalPolice is deprecated. It's also opaque because the prompt is configured manually in the OpenAI dashboard rather than checked into code and version controlled. This is problematic because I am maybe the only one who can change the prompt, and also because assistants are deprecated we can't use GPT-5, which might produce better results for cheaper than GPT-4o.

Solution

Migrate ProposalPolice to use the "normal" Chat Completions API rather than an assistant. Start using GPT-5.

[roryabraham]

From everything I've read, assistants was designed for the use case of back-and-forth conversations rather than for one-off responses to a prompt. So Chat Completions might have been more appropriate from the start 🤷🏼

[roryabraham]

Created #75624 to implement this.

[roryabraham]

latest context in https://expensify.slack.com/archives/C05LX9D6E07/p1765868797108589

[roryabraham]

Not a priority. Adding responses API in #77688. Had conversations API in the commit history there but ended up removing it