[FA-misc] Translation engine work

FictionArchive.Service.TranslationService.Tests/NanoGpt/NanoGptTranslationEngineTests.cs

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+using System.Net;
+using System.Text;
+using System.Text.Json;
+using FictionArchive.Service.TranslationService.Models.Enums;
+using FictionArchive.Service.TranslationService.Services.TranslationEngines.NanoGpt;
+using FictionArchive.Service.TranslationService.Tests.TestSupport;
+using FluentAssertions;
+using Microsoft.Extensions.Logging.Abstractions;
+using Polly;
+using Xunit;
+using Language = FictionArchive.Common.Enums.Language;
+
+namespace FictionArchive.Service.TranslationService.Tests.NanoGpt;
+
+public class NanoGptTranslationEngineTests
+{
+    private static (NanoGptTranslationEngine engine, TestHttpMessageHandler handler)
+        BuildEngine(Func<HttpRequestMessage, HttpResponseMessage> responder, string modelKey = "nanogpt-test", string modelId = "test-model")
+    {
+        var handler = new TestHttpMessageHandler(responder);
+        var httpClient = new HttpClient(handler) { BaseAddress = new Uri("https://nano-gpt.test/api/") };
+        var apiClient = new NanoGptApiClient(httpClient, NullLogger<NanoGptApiClient>.Instance);
+        var options = new NanoGptModelOptions { Key = modelKey, DisplayName = "Test", Model = modelId };
+        var pipeline = new ResiliencePipelineBuilder().Build(); // no-retry pipeline for deterministic tests
+        var engine = new NanoGptTranslationEngine(apiClient, options, pipeline, NullLogger<NanoGptTranslationEngine>.Instance);
+        return (engine, handler);
+    }
+
+    private static HttpResponseMessage JsonResponse(HttpStatusCode status, string json)
+    {
+        return new HttpResponseMessage(status)
+        {
+            Content = new StringContent(json, Encoding.UTF8, "application/json")
+        };
+    }
+
+    private const string SuccessJson = """
+        {
+          "choices": [{"message": {"role":"assistant","content":"안녕, 세상."}}],
+          "usage": {"prompt_tokens": 10, "completion_tokens": 5, "total_tokens": 15}
+        }
+        """;
+
+    [Fact]
+    public void Descriptor_ReflectsConfiguredOptions()
+    {
+        var (engine, _) = BuildEngine(_ => JsonResponse(HttpStatusCode.OK, SuccessJson),
+            modelKey: "nanogpt-gpt-4o");
+
+        engine.Descriptor.Key.Should().Be("nanogpt-gpt-4o");
+        engine.Descriptor.DisplayName.Should().Be("Test");
+    }
+
+    [Fact]
+    public async Task GetTranslation_OnSuccess_ReturnsTranslatedTextAndUsage()
+    {
+        var (engine, _) = BuildEngine(_ => JsonResponse(HttpStatusCode.OK, SuccessJson));
+
+        var result = await engine.GetTranslation("Hello, world.", Language.En, Language.Kr);
+
+        result.Status.Should().Be(TranslationRequestStatus.Success);
+        result.TranslatedText.Should().Be("안녕, 세상.");
+        result.OriginalText.Should().Be("Hello, world.");
+        result.From.Should().Be(Language.En);
+        result.To.Should().Be(Language.Kr);
+        result.TranslationEngineKey.Should().Be("nanogpt-test");
+        result.BilledCharacterCount.Should().Be(15u);
+    }
+
+    [Fact]
+    public async Task GetTranslation_PostsToChatCompletionsEndpoint()
+    {
+        var (engine, handler) = BuildEngine(_ => JsonResponse(HttpStatusCode.OK, SuccessJson));
+
+        await engine.GetTranslation("Hello", Language.En, Language.Kr);
+
+        handler.Requests.Should().HaveCount(1);
+        var request = handler.Requests[0];
+        request.Method.Should().Be(HttpMethod.Post);
+        request.RequestUri!.AbsoluteUri.Should().Be("https://nano-gpt.test/api/v1/chat/completions");
+    }
+
+    [Fact]
+    public async Task GetTranslation_SendsConfiguredModelAndTemperature()
+    {
+        var (engine, handler) = BuildEngine(
+            _ => JsonResponse(HttpStatusCode.OK, SuccessJson),
+            modelId: "chatgpt-4o-latest");
+
+        await engine.GetTranslation("Hello", Language.En, Language.Kr);
+
+        var body = JsonDocument.Parse(handler.RequestBodies[0]).RootElement;
+        body.GetProperty("model").GetString().Should().Be("chatgpt-4o-latest");
+        body.GetProperty("temperature").GetDouble().Should().Be(0.2);
+    }
+
+    [Fact]
+    public async Task GetTranslation_WrapsSourceTextInDelimitersAndIncludesSystemPrompt()
+    {
+        var (engine, handler) = BuildEngine(_ => JsonResponse(HttpStatusCode.OK, SuccessJson));
+
+        await engine.GetTranslation("Hello", Language.En, Language.Kr);
+
+        var body = JsonDocument.Parse(handler.RequestBodies[0]).RootElement;
+        var messages = body.GetProperty("messages");
+        messages.GetArrayLength().Should().Be(2);
+
+        var system = messages[0];
+        system.GetProperty("role").GetString().Should().Be("system");
+        var systemContent = system.GetProperty("content").GetString()!;
+        systemContent.Should().Contain("English").And.Contain("Korean");
+        systemContent.Should().Contain("<source_text>").And.Contain("Output only the translated text");
+
+        var user = messages[1];
+        user.GetProperty("role").GetString().Should().Be("user");
+        user.GetProperty("content").GetString().Should().Be("<source_text>\nHello\n</source_text>");
+    }
+
+    [Theory]
+    [InlineData(Language.En, "English")]
+    [InlineData(Language.Kr, "Korean")]
+    [InlineData(Language.Ch, "Chinese")]
+    [InlineData(Language.Ja, "Japanese")]
+    public async Task GetTranslation_RendersExpectedLanguageNamesInSystemPrompt(Language to, string expectedName)
+    {
+        var (engine, handler) = BuildEngine(_ => JsonResponse(HttpStatusCode.OK, SuccessJson));
+
+        await engine.GetTranslation("Hello", Language.En, to);
+
+        var body = JsonDocument.Parse(handler.RequestBodies[0]).RootElement;
+        var systemContent = body.GetProperty("messages")[0].GetProperty("content").GetString()!;
+        systemContent.Should().Contain(expectedName);
+    }
+
+    [Fact]
+    public async Task GetTranslation_On4xx_ReturnsFailedResultWithoutThrowing()
+    {
+        var (engine, _) = BuildEngine(_ => JsonResponse(HttpStatusCode.BadRequest,
+            """{"error":"bad model"}"""));
+
+        var result = await engine.GetTranslation("Hello", Language.En, Language.Kr);
+
+        result.Status.Should().Be(TranslationRequestStatus.Failed);
+        result.TranslatedText.Should().BeNull();
+        result.TranslationEngineKey.Should().Be("nanogpt-test");
+        result.OriginalText.Should().Be("Hello");
+    }
+
+    [Fact]
+    public async Task GetTranslation_On5xx_ReturnsFailedResultWithoutThrowing()
+    {
+        var (engine, _) = BuildEngine(_ => JsonResponse(HttpStatusCode.InternalServerError, "{}"));
+
+        var result = await engine.GetTranslation("Hello", Language.En, Language.Kr);
+
+        result.Status.Should().Be(TranslationRequestStatus.Failed);
+        result.TranslatedText.Should().BeNull();
+    }
+
+    [Fact]
+    public async Task GetTranslation_OnMalformedJson_ReturnsFailedResult()
+    {
+        var (engine, _) = BuildEngine(_ => JsonResponse(HttpStatusCode.OK, "not json"));
+
+        var result = await engine.GetTranslation("Hello", Language.En, Language.Kr);
+
+        result.Status.Should().Be(TranslationRequestStatus.Failed);
+        result.TranslatedText.Should().BeNull();
+    }
+
+    [Fact]
+    public async Task GetTranslation_OnEmptyChoices_ReturnsFailedResult()
+    {
+        const string emptyChoicesJson = """
+            {
+              "choices": [],
+              "usage": {"prompt_tokens": 0, "completion_tokens": 0, "total_tokens": 0}
+            }
+            """;
+        var (engine, _) = BuildEngine(_ => JsonResponse(HttpStatusCode.OK, emptyChoicesJson));
+
+        var result = await engine.GetTranslation("Hello", Language.En, Language.Kr);
+
+        result.Status.Should().Be(TranslationRequestStatus.Failed);
+        result.TranslatedText.Should().BeNull();
+    }
+}