Add MiniMax-M3 and MiniMax-M2.7 support

[octo-patch]

Reason: Add MiniMax-M3 and MiniMax-M2.7 to the existing compatible language-model path.

• Register model-specific context windows, tokenizer sources, and input modalities.
• Select the global or China endpoint with dedicated key and base URL overrides while preserving existing behavior.
• Enable multimodal prompt construction for MiniMax-M3 and add focused regression coverage.

Checks:

• python -m compileall -q llms agent tests/test_llms/test_minimax.py
• python -m pytest -q --confcutdir=tests/test_llms tests/test_llms/test_minimax.py
• git diff HEAD --check