Demo: Add request context to checkout logging

## Goal
Add request context to checkout logging while keeping local demo output readable.

## Context
The current logger in `demo/python_app_demo/app/logging_utils.py` emits the log level, logger name, and message, but it does not include request context such as a request ID.

## Acceptance criteria
- Define the request context fields required for the demo, including `request_id`.
- Add context-aware logging for the checkout path.
- Keep local output readable.
- Add or update tests that verify the required context appears in the log output.
- Run the complete test suite successfully.
- Keep the change limited to logging and directly related tests.

## Demo workflow
This issue is suitable for a second agent assignment after the first workflow is demonstrated. The agent should implement the change and open a pull request. A human must review the diff, check CI, approve the pull request, and own the merge decision.

## Note
If the required log format or fields remain unclear, pause assignment and refine this issue before asking an agent to implement it.

Start with demo/python_app_demo/app/logging_utils.py and trace how checkout logging is produced. Inspect any existing logging-related tests, clarify the required request-context fields and readable format, then update the logging and directly related tests; done means the required context appears in checkout output and the complete test suite passes.

Written by the indexing model from the issue text.