refactor: split monolithic server.py (800+ lines) into separate tool modules

Problem

minimax_mcp/server.py is a single 800+ line file containing all tool definitions, configuration, and the MCP server setup. This makes it hard to navigate, test, and maintain.

Current structure

minimax_mcp/
├── __init__.py      # 2 lines
├── __main__.py      # config generator
├── client.py        # API client (good, already separated)
├── const.py         # constants (good, already separated)
├── exceptions.py    # exceptions (good, already separated)
├── server.py        # 800+ lines — ALL tools + config + server setup
└── utils.py         # utilities (good, already separated)

Proposed structure

minimax_mcp/
├── __init__.py
├── __main__.py
├── client.py
├── const.py
├── exceptions.py
├── server.py        # ~50 lines — MCP setup, config, imports tools
├── utils.py
└── tools/
    ├── __init__.py  # exports all tool functions
    ├── audio.py     # text_to_audio, play_audio, list_voices
    ├── video.py     # generate_video, query_video_generation
    ├── image.py     # text_to_image
    ├── music.py     # music_generation
    └── voice.py     # voice_clone, voice_design

Benefits

• Each tool module is independently testable
• Easier to add new tools without touching a massive file
• Clear separation of concerns
• Reduces merge conflicts when multiple contributors work on different tools

Implementation notes

• Each tool module receives mcp, api_client, base_path, resource_mode as module-level or injected dependencies
• server.py imports and registers all tools
• No behavior change — purely structural refactor

Acceptance criteria

• server.py reduced to <100 lines
• Each tool in its own module
• All existing tests pass
• uvx minimax-mcp works identically

Start by reading minimax_mcp/server.py and the existing project tests to map the current tool definitions, configuration, and MCP setup. Separate the tools into the proposed modules under minimax_mcp/tools/ while preserving behavior, then run all existing tests and verify that uvx minimax-mcp works identically; done means server.py is under 100 lines and every tool has its own module.