[Bug]: ChildConfig.is_virtual is exposed to users

[zhongkechen]

Expected Behavior

ChildConfig.is_virtual is supposed to be used only internally by concurrency operations like map or parallel.

Actual Behavior

ChildConfig.is_virtual is accidentally exposed to users, which can be used with run_in_child_context operation

Steps to Reproduce

N/A

SDK Version

1.4-dev

Python Version

3.14

Is this a regression?

No

Last Working Version

No response

Additional Context

No response