fix(minimax): align request body with selected transport

[rixzkiye]

Summary

• select multi-endpoint provider transport from model-preferred target format when present
• keep translated request body, endpoint, and authentication protocol aligned
• avoid injecting OpenAI-only reasoning_content into Anthropic Messages requests
• add MiniMax global/China, M2.7/M3, and reasoning-injection regression coverage

Problem

MiniMax-M3 declares Claude as its target format. For OpenAI-format clients, transport selection previously used client source format while translation used model target format. This could send an Anthropic-shaped body and tool schema to MiniMax Chat Completions, causing 400 invalid tool type responses.

Verification

• cd tests && npx vitest run unit/minimax-transport-protocol.test.js unit/reasoningContentInjector.test.js unit/openai-to-claude-tools-no-type.test.js — 22 tests passed
• npx eslint open-sse/executors/default.js open-sse/handlers/chatCore.js open-sse/services/provider.js tests/unit/minimax-transport-protocol.test.js
• git diff --check origin/master...HEAD
• production Next.js build passed during implementation verification

No live MiniMax credentials were used; protocol behavior is covered with focused unit tests.

Related issues

Fixes #1939
Fixes #2435
Fixes #2509