Unit tests for `POST /chat`

[codebestia]

Create apps/ai_agent/tests/test_chat.py covering: missing API key returns 500, valid request returns reply, system prompt contains context, missing message returns 422, missing conversation_id returns 422, and correct model used.

Acceptance criteria:

• 6 tests, all pass
• mock_openai fixture used throughout (no direct OpenAI calls)
• System prompt content verified, not just presence

[Oyintarede]

@Oyintarede has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

Hi, I would like to work on this issue.

ℹ️ Repo Maintainers: To accept this application, review their application or assign @Oyintarede to this issue.

[drips-wave]

Congratulations, @Oyintarede! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on June 30, 2026.

🧑‍💻 @Oyintarede: Please resolve the issue such that the repo's maintainers have enough time to review your contribution before the due date. You'll earn Points for completing the issue on-time, which will make you eligible for a share of the Stellar Wave Program's reward pool.

Warning

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