feat(moderation): implement image content safety check

[Rader]

Summary:

• Implements image content safety checks by integrating Alibaba Cloud's PassImageURLCheck API to detect sensitive media, including support for both public URLs and private repository streams.
• Introduces a unified FileCheckContext to standardize file checking across all types and adds a configurable environment switch (STARHUB_SERVER_SENSITIVE_CHECK_IMAGE_ENABLE) to enable/disable the feature.
• Consolidates image file extension definitions into a single source of truth and ensures proper context cancellation handling with retry logic to prevent resource leaks.

[Rader]

Depends on #1266