cleanup() for cross_validate

[tcuongd]

#2668

[Copilot]

Pull Request Overview

This PR implements a cleanup() method to remove temporary files after model fitting, notably for CmdStan backends. Key changes include:

• Adding a cleanup method stub and an implemented version in python/prophet/models.py.
• Introducing the IStanBackend type in python/prophet/forecaster.py.
• Updating python/prophet/diagnostics.py to call the newly added cleanup() method on the stan_backend.

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
python/prophet/models.py	Introduces two cleanup() methods; one is a stub and another with logic.
python/prophet/forecaster.py	Adds the stan_backend attribute and IStanBackend import.
python/prophet/diagnostics.py	Calls cleanup() on stan_backend and adds type annotation to prophet_copy.