Add filesystem integration tests for ProjectBrowserClients

[elkraneo]

Summary

Add integration tests for FileOperationsClient and AssetDiscoveryClient that exercise actual filesystem operations using a temporary directory.

Motivation

The current unit tests mock the clients to test reducer logic. Integration tests would catch edge cases in the actual file operations:

• Name collision handling (uniqueURL)
• Permission errors
• Symlink handling
• Unicode filenames
• Case sensitivity on different filesystems

Proposed Tests

FileOperationsClient

@Test func move_handlesNameCollision() async throws {
    let temp = FileManager.default.temporaryDirectory.appending(component: UUID().uuidString)
    defer { try? FileManager.default.removeItem(at: temp) }
    
    // Setup: create file and folder with existing file of same name
    try FileManager.default.createDirectory(at: temp, withIntermediateDirectories: true)
    let source = temp.appending(component: "test.usda")
    let folder = temp.appending(component: "Folder")
    let existing = folder.appending(component: "test.usda")
    try "source".write(to: source, atomically: true, encoding: .utf8)
    try FileManager.default.createDirectory(at: folder, withIntermediateDirectories: false)
    try "existing".write(to: existing, atomically: true, encoding: .utf8)
    
    // Test
    let client = FileOperationsClient.liveValue
    let result = try await client.move([source], folder)
    
    // Should create "test 2.usda" due to collision
    #expect(result[source]?.lastPathComponent == "test 2.usda")
}

AssetDiscoveryClient

@Test func discover_findsNestedAssets() async throws {
    // Create a temp .rkassets structure with nested folders
    // Verify recursive scanning works correctly
}

Acceptance Criteria

• Tests use isolated temp directories (no interference between runs)
• Cleanup via defer even on test failure
• Cover: create, rename, move, delete, duplicate, import
• Cover: discovery with empty folders, nested folders, hidden files