feat: add OpenAI-compatible prompt expander (MiniMax, OpenAI, etc.)

[octo-patch]

Summary

Adds a third prompt extension backend (OpenAICompatPromptExpander) that works with any OpenAI-compatible LLM API, complementing the existing DashScope and local Qwen backends.

• By default uses MiniMax (MiniMax-M2.7 at https://api.minimax.io/v1), but the base URL and model can be overridden for OpenAI, DeepSeek, or any other OpenAI-compatible provider
• Supports both text-only (extend) and image+text (extend_with_img) prompt extension via base64 encoding
• Includes think-tag stripping for reasoning models and configurable retries
• New --prompt_extend_method openai_compatible CLI option in generate.py

Changes

File	Description
phantom_wan/utils/prompt_extend.py	New OpenAICompatPromptExpander class
generate.py	Wire up openai_compatible method + import
README.md	Usage docs for MiniMax/OpenAI-compatible prompt extension
tests/test_openai_compat_prompt_expander.py	29 unit tests
tests/test_openai_compat_integration.py	3 live integration tests

Usage

export MINIMAX_API_KEY="your-key"
python generate.py --task s2v-1.3B --size 832*480 \
  --ckpt_dir ./Wan2.1-T2V-1.3B \
  --phantom_ckpt ./Phantom-Wan-Models/Phantom-Wan-1.3B.pth \
  --ref_image "examples/ref1.png,examples/ref2.png" \
  --prompt "A girl playing with a dog" \
  --use_prompt_extend --prompt_extend_method openai_compatible

Test plan

• 29 unit tests pass (mocked OpenAI client)
• 3 integration tests pass (live MiniMax API)
• Existing DashScope/Qwen codepaths unaffected (no changes to existing classes)