[Upstream] feat: add MiniMax M2.5 models and update to OpenAI-compatible API

[Sagit-chu]

Synced from upstream PR: songquanpeng#2362

Summary

Add support for MiniMax's latest M2.5 model family and migrate to the standard OpenAI-compatible API endpoint.

Changes

• New models: Add MiniMax-M2.5 and MiniMax-M2.5-highspeed to the MiniMax channel model list
• API endpoint update: Migrate default base URL from legacy api.minimax.chat to api.minimax.io (OpenAI-compatible endpoint)
• Simplified routing: Remove custom URL routing for MiniMax; now uses standard OpenAI /v1/chat/completions endpoint
• Pricing: Add input/output pricing ratios for M2.5 models ($0.3/$1.2 per M tokens for M2.5, $0.6/$2.4 for M2.5-highspeed)
• Backward compatibility: Existing abab-series models remain in the model list

Model Details

Model	Input Price	Output Price	Context Window
MiniMax-M2.5	$0.3 / M tokens	$1.2 / M tokens	204K
MiniMax-M2.5-highspeed	$0.6 / M tokens	$2.4 / M tokens	204K

API Documentation

• OpenAI Compatible API: https://platform.minimax.io/docs/api-reference/text-openai-api