Add MiniMax prompt enhancer configuration

[octo-patch]

Reason: Add MiniMax prompt-enhancement support with configurable global and CN endpoints.

Summary:

• configure the prompt enhancer with MiniMax API key, region, base URL override, and supported text/image model settings
• route text prompt enhancement through MiniMax-M3 by default while allowing MiniMax-M2.7
• route image prompt enhancement through MiniMax-M3
• add focused unit coverage for endpoint selection and prompt request model routing

Checks:

• python3 -m unittest tests.test_prompt_enhancer
• python3 -m py_compile longcat_video/utils/prompt_enhancer.py tests/test_prompt_enhancer.py
• git diff --check
• Required secret scan