Implemented Translator for LibreTranslate (#739)

* Implemented Translator for LibreTranslate

* LibreTranslate: Added option for number of alternatives

The API offers the function to provide alternative translations, so it would make sense to offer this to the user as well.

* Revert unrelated changes

* Added missing pragma

* Fixed build of unit test

* Removed obsolete comment

* Added "/translate" to the API url

* Changed URI to point to the github repo

Author: sven-n

src/ResXManager.Tests/Model/LibreTranslateTranslatorTests.cs

@@ -0,0 +1,142 @@
+namespace ResXManager.Tests.Model;
+
+using System;
+using System.Globalization;
+using System.Linq;
+using System.Net.Http;
+using System.Threading;
+using System.Threading.Tasks;
+
+using Xunit;
+
+public class LibreTranslateTranslatorTests
+{
+    [Fact]
+    public void LibreTranslateParsesResponseCorrectly()
+    {
+        const string input = "{\"translatedText\":\"¡Hola!\"}";
+
+        var result = Translators.LibreTranslateTranslator.ParseResponse(input).First();
+
+        Assert.Equal("¡Hola!", result);
+    }
+
+    [Fact]
+    public void LibreTranslateParsesResponseWithDetectedLanguageCorrectly()
+    {
+        const string input = "{\"detectedLanguage\":{\"confidence\":90.0,\"language\":\"fr\"},\"translatedText\":\"Hello!\"}";
+
+        var result = Translators.LibreTranslateTranslator.ParseResponse(input).First();
+
+        Assert.Equal("Hello!", result);
+    }
+
+    [Fact]
+    public void LibreTranslateParsesEmptyResponseCorrectly()
+    {
+        const string input = "{}";
+
+        var result = Translators.LibreTranslateTranslator.ParseResponse(input);
+
+        Assert.Null(result);
+    }
+
+    [Fact]
+    public void LibreTranslateParsesNullTranslatedTextCorrectly()
+    {
+        const string input = "{\"translatedText\":null}";
+
+        var result = Translators.LibreTranslateTranslator.ParseResponse(input);
+
+        Assert.Null(result);
+    }
+
+    [Fact]
+    public void LibreTranslateParsesResponseWithAlternativesCorrectly()
+    {
+        const string input = "{\"translatedText\":\"Hi\",\"alternatives\":[\"Hello\",\"Hey\"]}";
+
+        var result = Translators.LibreTranslateTranslator.ParseResponse(input);
+
+        Assert.Equal(["Hi", "Hello", "Hey"], result);
+    }
+
+    [Fact]
+    public void LibreTranslateThrowsOnBadJson()
+    {
+        const string input = """
+                             {"translatedText":
+                             """;
+
+        Assert.ThrowsAny<Exception>(() => Translators.LibreTranslateTranslator.ParseResponse(input));
+    }
+
+    #region Integration Tests (require local LibreTranslate instance on localhost:5000)
+
+    private const string LocalLibreUrl = "http://localhost:5000";
+
+    private static async Task<bool> IsLibreTranslateAvailableAsync()
+    {
+        try
+        {
+            using var httpClient = new HttpClient { Timeout = TimeSpan.FromSeconds(2) };
+            var response = await httpClient.GetAsync($"{LocalLibreUrl}/languages");
+            return response.IsSuccessStatusCode;
+        }
+        catch
+        {
+            return false;
+        }
+    }
+
+    [Fact]
+    public async Task TranslateEnglishToGerman_ReturnsTranslatedText()
+    {
+        if (!await IsLibreTranslateAvailableAsync())
+        {
+            // Skip test if LibreTranslate is not running locally
+            return;
+        }
+
+        var result = await Translators.LibreTranslateTranslator.TranslateAsync(
+            $"{LocalLibreUrl}/translate",
+            apiKey: null,
+            text: "Good morning",
+            sourceLanguage: CultureInfo.GetCultureInfo("en"),
+            targetLanguage: CultureInfo.GetCultureInfo("de"),
+            alternatives: null,
+            cancellationToken: CancellationToken.None);
+
+        Assert.NotNull(result);
+        Assert.NotEmpty(result);
+        Assert.NotEmpty(result[0]);
+
+        // "Good morning" in German is typically "Guten Morgen"
+        Assert.Contains("Guten Morgen", result[0], StringComparison.OrdinalIgnoreCase);
+    }
+
+    [Fact]
+    public async Task TranslateWithCancellation_ThrowsOperationCanceledException()
+    {
+        if (!await IsLibreTranslateAvailableAsync())
+        {
+            // Skip test if LibreTranslate is not running locally
+            return;
+        }
+
+        using var cts = new CancellationTokenSource();
+        cts.Cancel();
+
+        await Assert.ThrowsAnyAsync<OperationCanceledException>(async () =>
+            await Translators.LibreTranslateTranslator.TranslateAsync(
+                LocalLibreUrl,
+                apiKey: null,
+                text: "Hello",
+                sourceLanguage: CultureInfo.GetCultureInfo("en"),
+                targetLanguage: CultureInfo.GetCultureInfo("es"),
+                alternatives: null,
+                cancellationToken: cts.Token));
+    }
+
+    #endregion
+}