Refactor: Rename DocumentOperator to DocumentService

Updated project and solution files to rename and restructure
`DocumentOperator` to `DocumentService`. Modified project
references in `DocumentService.API.csproj`, `DocumentService.Application.csproj`,
and `DocumentService.Infrastructure.csproj` to reflect the new
naming convention. Updated `DocumentService.sln` to remove
old project references, add new ones, and adjust configuration
and nested project mappings accordingly. This change aligns
the project structure with the new naming convention.

DocumentService.Tests/Unit/Application/Features/ValidatePdfA/ValidatePdfAQueryHandlerTests.cs

@@ -0,0 +1,129 @@
+using DocumentService.Application.Common.DTOs;
+using DocumentService.Application.Common.Interfaces;
+using DocumentService.Application.ValidatePdfA.Queries;
+using DocumentService.Domain.Common.Exceptions;
+using FluentAssertions;
+using Moq;
+using Xunit;
+
+namespace DocumentService.Tests.Unit.Application.Features.ValidatePdfA;
+
+public class ValidatePdfAQueryHandlerTests
+{
+    private readonly Mock<IPdfProcessor> _mockPdfProcessor;
+    private readonly ValidatePdfAQueryHandler _handler;
+
+    public ValidatePdfAQueryHandlerTests()
+    {
+        _mockPdfProcessor = new Mock<IPdfProcessor>();
+        _handler = new ValidatePdfAQueryHandler(_mockPdfProcessor.Object);
+    }
+
+    [Fact]
+    public async Task Handle_ValidCompliantPdfA_ReturnsCompliantResult()
+    {
+        // Arrange
+        var query = new ValidatePdfAQuery { PdfStream = new MemoryStream("%PDF"u8.ToArray()) };
+        var expected = new PdfAValidationResult
+        {
+            IsValid = true, PdfVersion = "1.7", PageCount = 3, FileSize = 2048,
+            Encrypted = false, PdfAVersion = "PDF/A-3b", PdfACompliant = true,
+            Errors = [], Warnings = []
+        };
+
+        _mockPdfProcessor.Setup(x => x.ValidatePdfAAsync(It.IsAny<Stream>())).ReturnsAsync(expected);
+
+        // Act
+        var result = await _handler.Handle(query, CancellationToken.None);
+
+        // Assert
+        result.IsValid.Should().BeTrue();
+        result.PageCount.Should().Be(3);
+        result.PdfVersion.Should().Be("1.7");
+        result.PdfAVersion.Should().Be("PDF/A-3b");
+        result.PdfACompliant.Should().BeTrue();
+        result.Encrypted.Should().BeFalse();
+        result.Errors.Should().BeEmpty();
+        result.Warnings.Should().BeEmpty();
+        _mockPdfProcessor.Verify(x => x.ValidatePdfAAsync(It.IsAny<Stream>()), Times.Once);
+    }
+
+    [Fact]
+    public async Task Handle_NonCompliantPdfA_ReturnsErrorsAndWarnings()
+    {
+        // Arrange
+        var query = new ValidatePdfAQuery { PdfStream = new MemoryStream("%PDF"u8.ToArray()) };
+        var expected = new PdfAValidationResult
+        {
+            IsValid = false, PdfVersion = "1.4", PageCount = 2, FileSize = 1024,
+            Encrypted = false, PdfAVersion = null, PdfACompliant = false,
+            Errors = ["Missing XMP metadata", "Invalid color space"],
+            Warnings = ["Embedded font not subset"]
+        };
+
+        _mockPdfProcessor.Setup(x => x.ValidatePdfAAsync(It.IsAny<Stream>())).ReturnsAsync(expected);
+
+        // Act
+        var result = await _handler.Handle(query, CancellationToken.None);
+
+        // Assert
+        result.IsValid.Should().BeFalse();
+        result.PdfACompliant.Should().BeFalse();
+        result.PdfAVersion.Should().BeNull();
+        result.Errors.Should().HaveCount(2).And.Contain("Missing XMP metadata").And.Contain("Invalid color space");
+        result.Warnings.Should().HaveCount(1).And.Contain("Embedded font not subset");
+        _mockPdfProcessor.Verify(x => x.ValidatePdfAAsync(It.IsAny<Stream>()), Times.Once);
+    }
+
+    [Fact]
+    public async Task Handle_EncryptedPdf_ReturnsEncryptedFlagAndErrors()
+    {
+        // Arrange
+        var query = new ValidatePdfAQuery { PdfStream = new MemoryStream("%PDF"u8.ToArray()) };
+        var expected = new PdfAValidationResult
+        {
+            IsValid = false, PdfVersion = "1.7", PageCount = 1, FileSize = 512,
+            Encrypted = true, PdfAVersion = null, PdfACompliant = false,
+            Errors = ["PDF/A documents cannot be encrypted"],
+            Warnings = []
+        };
+
+        _mockPdfProcessor.Setup(x => x.ValidatePdfAAsync(It.IsAny<Stream>())).ReturnsAsync(expected);
+
+        // Act
+        var result = await _handler.Handle(query, CancellationToken.None);
+
+        // Assert
+        result.Encrypted.Should().BeTrue();
+        result.IsValid.Should().BeFalse();
+        result.PdfACompliant.Should().BeFalse();
+        result.Errors.Should().Contain("PDF/A documents cannot be encrypted");
+        _mockPdfProcessor.Verify(x => x.ValidatePdfAAsync(It.IsAny<Stream>()), Times.Once);
+    }
+
+    [Fact]
+    public async Task Handle_ProcessorThrowsBadRequest_PropagatesException()
+    {
+        // Arrange
+        var query = new ValidatePdfAQuery { PdfStream = new MemoryStream("%PDF"u8.ToArray()) };
+        _mockPdfProcessor.Setup(x => x.ValidatePdfAAsync(It.IsAny<Stream>()))
+            .ThrowsAsync(new BadRequestException("Invalid PDF format"));
+
+        // Act & Assert
+        var ex = await Assert.ThrowsAsync<BadRequestException>(
+            () => _handler.Handle(query, CancellationToken.None));
+        ex.Message.Should().Be("Invalid PDF format");
+    }
+
+    [Fact]
+    public async Task Handle_ProcessorThrowsGenericException_PropagatesException()
+    {
+        // Arrange
+        var query = new ValidatePdfAQuery { PdfStream = new MemoryStream("%PDF"u8.ToArray()) };
+        _mockPdfProcessor.Setup(x => x.ValidatePdfAAsync(It.IsAny<Stream>()))
+            .ThrowsAsync(new Exception("Unexpected processing error"));
+
+        // Act & Assert
+        await Assert.ThrowsAsync<Exception>(() => _handler.Handle(query, CancellationToken.None));
+    }
+}