feat(mcp): allow 'never' option for mcp_timeout to disable timeout

[Skyline-23]

Closes #8633

Summary

• Add "never" option to mcp_timeout config to allow disabling timeout for MCP tool calls
• Previously only positive integers (milliseconds) were supported

Changes

• config.ts: Update schema to accept "never" | number
• mcp/index.ts: Handle "never" by passing undefined to disable timeout

Usage

{
  "experimental": {
    "mcp_timeout": "never"
  }
}

Or with a specific timeout:

{
  "experimental": {
    "mcp_timeout": 60000
  }
}