Improve session handling and add API tests

[MyButtermilk]

Summary

• enforce Pipecat transcription requests to honor the path session_id even when payloads include a different value
• add pytest-based unit tests covering health, session creation, and transcription flows via a dummy session manager
• include pytest in backend requirements and test configuration helpers

Testing

• cd backend && pytest

---

Codex Task

[chatgpt-codex-connector]

💡 Codex Review

Here are some automated review suggestions for this pull request.

ℹ️ About Codex in GitHub

Your team has set up Codex to review pull requests in this repo. Reviews are triggered when you

• Open a pull request for review
• Mark a draft as ready
• Comment "@codex review".

If Codex has suggestions, it will comment; otherwise it will react with 👍.

Codex can also answer questions or update the PR. Try commenting "@codex address that feedback".