Implemented the FileProviderMock mock type in the tests target.

Tests/DocCMiddleware/Types/Mocks/FileProviderMock.swift

@@ -0,0 +1,102 @@
+// ===----------------------------------------------------------------------===
+//
+// This source file is part of the Hummingbird DocC Middleware open source project
+//
+// Copyright (c) 2025 Röck+Cöde VoF. and the Hummingbird DocC Middleware project authors
+// Licensed under the EUPL 1.2 or later.
+//
+// See LICENSE for license information
+// See CONTRIBUTORS for the list of Hummingbird DocC Middleware project authors
+//
+// ===----------------------------------------------------------------------===
+
+import protocol Hummingbird.FileProvider
+import protocol Hummingbird.RequestContext
+
+import struct Foundation.Data
+import struct Foundation.UUID
+import struct Hummingbird.ResponseBody
+
+/// A mock that conforms to the `FileProvider` protocol.
+struct FileProviderMock {
+    
+    // MARK: Properties
+    
+    /// A type that identifies a sample file.
+    private let fileIdentifier: UUID?
+    
+    /// A flag that indicates whether a file should be loaded or not.
+    private let shouldLoadFile: Bool
+    
+    // MARK: Initializers
+    
+    /// Initializes this mock.
+    /// - Parameters:
+    ///   - fileIdentifier: A type that identifies a sample file, if any.
+    ///   - shouldLoadFile: A flag that indicates whether a file should be loaded or not.
+    init(
+        fileIdentifier: UUID? = nil,
+        shouldLoadFile: Bool = true
+    ) {
+        self.fileIdentifier = fileIdentifier
+        self.shouldLoadFile = shouldLoadFile
+    }
+    
+}
+
+// MARK: - FileProvider
+
+extension FileProviderMock: FileProvider {
+
+    // MARK: Type aliases
+
+    typealias FileAttributes = String
+    typealias FileIdentifier = String
+    
+    // MARK: Functions
+
+    func getFileIdentifier(_ path: String) -> String? {
+        fileIdentifier?.uuidString
+    }
+    
+    func getAttributes(id: String) async throws -> String? {
+        nil
+    }
+    
+    func loadFile(
+        id: String,
+        context: some RequestContext
+    ) async throws -> ResponseBody {
+        guard shouldLoadFile else {
+            throw FileProviderMockError.fileNotLoaded
+        }
+
+        guard let content = fileIdentifier?.uuidString else {
+            return .init()
+        }
+        
+        return .init(byteBuffer: .init(
+            data: .init(content.utf8)
+        ))
+    }
+    
+    func loadFile(
+        id: String,
+        range: ClosedRange<Int>,
+        context: some RequestContext
+    ) async throws -> ResponseBody {
+        try await loadFile(
+            id: id,
+            context: context
+        )
+    }
+    
+}
+
+// MARK: - FileProviderMockError
+
+/// An error type that can only be thrown by the ``FileProviderMock`` mock.
+enum FileProviderMockError: Error {
+    /// An error encountered while mocking the loading of a file.
+    case fileNotLoaded
+}