Demo: Fix flaky checkout ID test through agent PR workflow

## Goal
Make the checkout ID test deterministic so it can run reliably in CI.

## Context
`demo/python_app_demo/tests/test_flaky_checkout_id.py` currently generates a random checkout ID and skips the test. Randomness means the assertion can pass or fail unpredictably.

## Acceptance criteria
- Replace uncontrolled randomness with deterministic test behavior.
- Remove the skip marker.
- Preserve the assertion that the checkout ID is even.
- Run the targeted test successfully.
- Run the complete test suite successfully.
- Keep the change limited to the test and directly related files.

## Demo workflow
This issue is intended to be assigned to a coding agent. The agent should implement the change and open a pull request. A human must review the diff, confirm CI is green, approve the pull request, and own the merge decision.

## Completion statement
At merge time, the human reviewer should say: “I checked the diff. I own this merge.”

Start with demo/python_app_demo/tests/test_flaky_checkout_id.py and run the targeted test to observe its current behavior. Make the checkout ID generation deterministic, remove the skip marker, preserve the even-ID assertion, and run both the targeted test and complete test suite successfully.

Written by the indexing model from the issue text.