Add MiniMax provider adapter for LLM Protocol V1

[octo-patch]

Reason: Add a MiniMax provider adapter for the vendor-neutral LLM Protocol V1 so the SDK supports MiniMax-M3 / MiniMax-M2.7 against the global and China endpoints.

Summary

The SDK exposes a concrete adapter contract (BaseLLMAdapter / LLMProtocolV1) for completions, streaming, supported models, context limits, and tool capabilities, but shipped no MiniMax adapter. This change adds a concrete MiniMaxAdapter that implements the contract against MiniMax's OpenAI-compatible chat completions surface.

Changes

• corpus_sdk/llm/providers/__init__.py: new package exporting MiniMaxAdapter, MINIMAX_MODELS, and MINIMAX_REGIONS.
• corpus_sdk/llm/providers/minimax.py: the adapter implementation.
  • Implements _do_capabilities, _do_complete, _do_stream, _do_count_tokens, and _do_health from BaseLLMAdapter.
  • Model registry (MINIMAX_MODELS) advertises MiniMax-M3 (1,000,000-token context; text/image/video input; adaptive/disabled thinking) and MiniMax-M2.7 (204,800-token context; text input; always-on thinking), with per-model pricing.
  • Regional endpoint configuration (MINIMAX_REGIONS) covers global_en (https://api.minimax.io/v1) and cn_zh (https://api.minimaxi.com/v1), including the Anthropic-compatible base URLs and docs roots.
  • Uses only the standard-library urllib stack (wrapped with asyncio.to_thread) so the package keeps zero runtime dependencies.
  • Pluggable transport callable lets callers and tests short-circuit network I/O.
  • Maps upstream HTTP statuses onto the normalized error taxonomy (BadRequest, AuthError, ResourceExhausted, Unavailable, TransientNetwork).
• tests/llm/test_minimax_adapter.py: 22 unit tests using a fake transport (no network) covering capabilities, regional endpoint selection, completion parsing, tool-call parsing, streaming SSE parsing, token counting, health, error mapping, and protocol conformance.

Checks

• pytest tests/llm/test_minimax_adapter.py — 22 passed (Python 3.12, pytest 8.4.2, pytest-asyncio 0.23.8).
• pytest tests/llm/test_capabilities_shape.py tests/llm/test_complete_basic.py — 24 passed (existing conformance tests unaffected).
• black --line-length 100 --skip-string-normalization applied to the new files.