Python: Docs: clarify Python Foundry Hosting session isolation configuration

[moonbox3]

Follow-up

Add docs guidance for the Python Foundry Hosting session isolation behavior introduced in PR #6536.

The docs should clarify:

• hosted workflow checkpoint state is scoped by a hosted session identity context by default
• production Foundry hosting is expected to provide the platform isolation keys on protocol requests
• self-hosted or local-only deployments that do not provide those keys must explicitly configure their own provider or opt into strict_session_isolation=False
• when strict_session_isolation=False is appropriate, and when it should not be used

Keep this as documentation-only follow-up; no code changes are required for this issue.