feat: add manage_asset_store tool for Asset Store package operations

Add a new MCP tool that enables AI assistants to list, download, and
import Unity Asset Store packages programmatically.

## Actions

- **check_auth**: Verify Unity account login status
- **list_purchases**: List all purchased Asset Store packages with
  demand-driven pagination via MyAssetsPage.LoadMore
- **download**: Download a package by product ID (awaits completion)
- **import**: Import a downloaded .unitypackage into the project

## Architecture

Uses reflection into Unity's internal Package Manager APIs
(UnityEditor.PackageManager.UI.Internal.*), following the same pattern
as the existing FrameDebuggerOps profiler tool. A hidden (never-shown)
PackageManagerWindow instance provides access to all internal services
without any visible UI side effects.

Key internal types accessed:
- AssetStoreDownloadManager (download via IEnumerable<long> overload)
- AssetStoreCache/AssetStoreLocalInfo (package path lookup)
- AssetStoreListOperation (pagination progress tracking)
- PackageDatabase (package metadata)
- PaginatedVisualStateList (accurate purchase count/pagination)

## Implementation Details

- Async handlers using TaskCompletionSource + EditorApplication.update
  (same pattern as RefreshUnity.cs)
- Lazy type/method resolution cached across calls, cleared on domain reload
- Graceful degradation: returns clear error messages if internal APIs
  are unavailable or changed in a different Unity version
- Download completion verified after await — returns ErrorResponse on
  failure or timeout
- Tested on Unity 6000.4.0f1 (Linux)

## Files

- MCPForUnity/Editor/Tools/ManageAssetStore.cs (C# handler)
- Server/src/services/tools/manage_asset_store.py (Python MCP tool)
- Server/src/cli/commands/asset_store.py (CLI commands)
- Server/src/cli/main.py (CLI registration, +1 line)
- Server/tests/test_manage_asset_store.py (14 tests)

Co-Authored-By: Claude Opus 4.6 (1M context) <[email protected]>

Author: Sebastian Muehr