[ME][4/7] Clean up character imports and MainTex refreshes on disable

[tomTom1010EEE]

Description

• Cancel queued texture imports and MainTex refreshes when the character controller is disabled.
• Cancel late import callbacks and prevent old refresh cleanup from affecting new requests after re-enabling.
• Keep save formats and public APIs unchanged.

Motivation and Context

Prevent imports from remaining pending while disabled and MainTex refreshes from becoming stuck after interruption.

How Has This Been Tested?

• Release builds passed for API, KK, KKS, EC, AI, HS2 and PH.

Screenshots (if appropriate):

N/A.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)