Add MiniMax provider preset with global and China endpoints to LLMClient

[octo-patch]

Reason: Add MiniMax as a native provider in LLMClient with global and China endpoint presets and MiniMax-M3 / MiniMax-M2.7 model support.

Changes

• Add to mapping the MiniMax provider to its documented OpenAI Chat Completions base URLs per region ( -> , -> ).
• Add and kwargs (and matching / env vars) to , with resolving the base URL from an explicit value, then env, then a provider preset, then the OpenAI default. An explicit keeps winning, and the existing image-capable chat request path is unchanged.
• Add covering both regional endpoints, both MiniMax-M3 and MiniMax-M2.7, case-insensitive provider lookup, explicit-URL override, region fallback, env-based configuration, the assembled URL, and the unchanged image helper.

Verification

• Ran (11 passed).

[octo-patch]

Updated MiniMax provider support with Anthropic Messages endpoints for both regions, current model defaults, and image-content conversion. I ran Python 3.9/3.14 compilation, git diff checks, and offline request/response smoke tests; the focused pytest suite was not run because this worker lacks pytest and rdkit.