Implement Package Remove tool for Unity MCP with domain reload handling

[Copilot]

This PR implements the Package Remove tool for Unity MCP, allowing AI assistants to programmatically remove Unity packages from a project's manifest.json file.

Key Features

Core Functionality:

• Removes multiple Unity packages in a single operation
• Modifies /Packages/manifest.json using Unity-compatible System.Text.Json
• Returns individual success/warning results for each package on separate lines
• Automatically refreshes AssetDatabase after package removal

Async Operation Handling:

• Uses ResponseCallTool.Processing() status during AssetDatabase refresh operations
• Implements PackageRemovalTracker with [InitializeOnLoad] to survive domain reloads
• Properly handles both immediate completion and domain reload scenarios
• Notifies MCP server when operations complete using McpPluginUnity.NotifyToolRequestCompleted()

Robust Error Handling:

• Validates RequestID and package array parameters
• Detects missing or malformed manifest.json files
• Monitors compilation status after domain reload
• Provides detailed error messages for troubleshooting

Implementation Details

The tool follows the established patterns from TestRunner.Run.cs:

• Uses MainThread.Instance.RunAsync() for thread safety
• Implements proper [RequestID] parameter handling
• Returns ResponseCallTool with appropriate status codes
• Handles domain reload completion via AssemblyReloadEvents.afterAssemblyReload

Example Usage:

// Remove multiple packages
var result = await Tool_Package.Remove(
    new[] { "com.unity.postprocessing", "org.nuget.system.text.json" },
    "request-123"
);

// Returns results like:
// [Success] Package com.unity.postprocessing removed
// [Warning] Package org.nuget.system.text.json not found

Testing

Comprehensive unit tests cover:

• RequestID validation and error scenarios
• Package removal tracker functionality across domain reloads
• JSON manipulation and file operations
• Edge cases like empty package arrays and compilation errors

Files Added

• Package.cs - Base class with error definitions
• Package.Remove.cs - Main implementation (283 lines)
• TestToolPackageRemove.cs - Unit tests (192 lines)
• Package.Remove.md - Usage documentation
• Unity .meta files for proper asset management

The implementation enables AI assistants to safely manage Unity project dependencies, clean up unused packages, and resolve package conflicts programmatically.

Fixes #228.

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