MCP tool descriptions should list available models and config options

The codex MCP tool descriptions are too sparse for AI agents to use effectively without external documentation.

Current state:

• model field says "e.g. 'gpt-5.2', 'gpt-5.2-codex'" but doesn't list available options
• config field doesn't mention model_reasoning_effort or valid values

Suggested improvement:

• List available models in the model description: gpt-5.2-codex, gpt-5.1-codex-max, gpt-5.1-codex, gpt-5-codex, gpt-5
• Document common config keys in the config description, especially model_reasoning_effort (minimal, low, medium, high, xhigh)

This would allow AI agents using the MCP to know what values are valid without needing to web search or consult external docs.

File: codex-rs/mcp-server/src/codex_tool_config.rs