Support MiniMax image-to-image references

[octo-patch]

Reason: MiniMax image generation did not expose the subject reference input required for image-to-image requests.

Summary

• Add an optional subject reference image field to the MiniMax image model settings.
• Normalize image URLs or data URLs into the documented subject_reference payload while preserving structured references.
• Keep text-to-image payloads unchanged when no reference is configured.
• Add focused unit coverage for configured and direct reference inputs.

Checks

• python3 -m unittest tests/models_provider/test_minimax_tti.py -v
• python3 -m py_compile apps/models_provider/impl/minimax_model_provider/model/tti.py apps/models_provider/impl/minimax_model_provider/credential/tti.py tests/models_provider/test_minimax_tti.py
• uvx --from 'ruff==0.15.12' ruff check apps/models_provider/impl/minimax_model_provider/model/tti.py apps/models_provider/impl/minimax_model_provider/credential/tti.py tests/models_provider/test_minimax_tti.py
• git diff --check origin/v2...HEAD