Add JSON-serializable output models for all Apple APIs

[rmarinho]

Summary

Define structured, JSON-serializable result types for all Apple environment and simulator management APIs. This supports the --json global option from the CLI spec.

Requirements

Every API should return a typed result that can be serialized to JSON. The output contract should include:

Standard envelope

All JSON output follows a consistent structure:

{ "type": "result", "command": "apple.check", "data": { ... } }
{ "type": "progress", "message": "Downloading iOS 18.2 runtime...", "percent": 45 }
{ "type": "error", "code": "XCODE_NOT_FOUND", "message": "..." }

Model types needed

• XcodeInfo — version, path, build number, bundled SDKs, isSelected
• SimulatorInfo — name, UDID, state, runtime, deviceType
• RuntimeInfo — platform, version, build, identifier, size, source
• EnvironmentCheckResult — xcode, clt, runtimes, platforms, overall status
• ProgressReport — message, percent, step
• ErrorResult — code, message, details

Design Notes

• Use System.Text.Json source generators for AOT-friendly serialization
• Follow the naming conventions from the CLI spec (camelCase JSON properties)
• These models are the contract between the library and the CLI — keep them stable
• Consider placing in a separate namespace (e.g., Xamarin.MacDev.Models) for clean separation

Reference

• CLI spec PR