test: add Application handler tests for all missing features

New test files (4 tests each — 2 success + 2 failure):
- MergePdfsCommandHandlerTests: TwoStreams, WithPageRanges, BadRequest, GenericException
- AddAnnotationHandlerTests: TextMarkup, FreeText, BadRequest (page OOB), GenericException
- AddStampHandlerTests: TextStamp, PredefinedStamp, BadRequest (opacity), GenericException
- ExtractPdfAttachmentsHandlerTests: ReturnsZip, NotFoundException, BadRequest, GenericException
- ConvertPdfAHandlerTests (ConvertTo + ConvertFrom):
  ConvertTo: ValidLevel, NotImplemented, BadRequest
  ConvertFrom: ValidPdfA, BadRequest, GenericException

DocumentOperator.Tests/Unit/Application/Features/AddStamp/AddStampHandlerTests.cs

@@ -0,0 +1,133 @@
+using DocumentService.Application.AddStamp;
+using DocumentService.Application.Common.Interfaces;
+using DocumentService.Domain.Common.Exceptions;
+using DocumentService.Domain.Models.ValueObjects;
+using FluentAssertions;
+using Moq;
+using Xunit;
+
+namespace DocumentService.Tests.Unit.Application.Features.AddStamp;
+
+public class AddStampHandlerTests
+{
+    private readonly Mock<IPdfProcessor> _mockProcessor;
+    private readonly AddStampHandler _handler;
+    private static readonly byte[] FakePdfBytes = "%PDF"u8.ToArray();
+
+    public AddStampHandlerTests()
+    {
+        _mockProcessor = new Mock<IPdfProcessor>();
+        _handler = new AddStampHandler(_mockProcessor.Object);
+    }
+
+    [Fact]
+    public async Task Handle_TextStamp_ReturnsStampedBytes()
+    {
+        // Arrange
+        var command = new AddStampCommand
+        {
+            PdfStream = new MemoryStream(FakePdfBytes),
+            StampType = StampType.Text,
+            Position = (100, 200),
+            Text = "CONFIDENTIAL"
+        };
+        var expected = new byte[] { 0x25, 0x50, 0x44, 0x46, 0x03 };
+        _mockProcessor.Setup(x => x.AddStampAsync(
+                It.IsAny<Stream>(), StampType.Text, null,
+                It.IsAny<(double, double)>(), null,
+                AnnotationOrigin.BottomLeft, "CONFIDENTIAL",
+                null, null, null, null, null,
+                StampPlacement.Foreground, null, null))
+            .ReturnsAsync(expected);
+
+        // Act
+        var result = await _handler.Handle(command, CancellationToken.None);
+
+        // Assert
+        result.Should().BeEquivalentTo(expected);
+        _mockProcessor.Verify(x => x.AddStampAsync(
+            It.IsAny<Stream>(), StampType.Text, null,
+            It.IsAny<(double, double)>(), null,
+            AnnotationOrigin.BottomLeft, "CONFIDENTIAL",
+            null, null, null, null, null,
+            StampPlacement.Foreground, null, null), Times.Once);
+    }
+
+    [Fact]
+    public async Task Handle_PredefinedStamp_PassesPredefinedTypeToProcessor()
+    {
+        // Arrange
+        var command = new AddStampCommand
+        {
+            PdfStream = new MemoryStream(FakePdfBytes),
+            StampType = StampType.Predefined,
+            Position = (50, 50),
+            PredefinedType = PredefinedStampType.Approved,
+            Opacity = 0.8
+        };
+        var expected = new byte[] { 0x25, 0x50, 0x44, 0x46, 0x04 };
+        _mockProcessor.Setup(x => x.AddStampAsync(
+                It.IsAny<Stream>(), StampType.Predefined, null,
+                It.IsAny<(double, double)>(), null,
+                AnnotationOrigin.BottomLeft, null, null, null, null,
+                0.8, null, StampPlacement.Foreground, null, PredefinedStampType.Approved))
+            .ReturnsAsync(expected);
+
+        // Act
+        var result = await _handler.Handle(command, CancellationToken.None);
+
+        // Assert
+        result.Should().BeEquivalentTo(expected);
+    }
+
+    [Fact]
+    public async Task Handle_ProcessorThrowsBadRequest_PropagatesException()
+    {
+        // Arrange
+        var command = new AddStampCommand
+        {
+            PdfStream = new MemoryStream(FakePdfBytes),
+            StampType = StampType.Text,
+            Position = (0, 0),
+            Text = "DRAFT",
+            Opacity = 2.0  // invalid
+        };
+        _mockProcessor.Setup(x => x.AddStampAsync(
+                It.IsAny<Stream>(), It.IsAny<StampType>(), It.IsAny<int[]?>(),
+                It.IsAny<(double, double)>(), It.IsAny<(double, double)?>(),
+                It.IsAny<AnnotationOrigin>(), It.IsAny<string?>(), It.IsAny<string?>(),
+                It.IsAny<double?>(), It.IsAny<string?>(), It.IsAny<double?>(),
+                It.IsAny<double?>(), It.IsAny<StampPlacement>(),
+                It.IsAny<byte[]?>(), It.IsAny<PredefinedStampType?>()))
+            .ThrowsAsync(new BadRequestException("Opacity must be between 0.0 and 1.0"));
+
+        // Act & Assert
+        var ex = await Assert.ThrowsAsync<BadRequestException>(
+            () => _handler.Handle(command, CancellationToken.None));
+        ex.Message.Should().Contain("Opacity");
+    }
+
+    [Fact]
+    public async Task Handle_ProcessorThrowsGenericException_PropagatesException()
+    {
+        // Arrange
+        var command = new AddStampCommand
+        {
+            PdfStream = new MemoryStream(FakePdfBytes),
+            StampType = StampType.Text,
+            Position = (0, 0),
+            Text = "TEST"
+        };
+        _mockProcessor.Setup(x => x.AddStampAsync(
+                It.IsAny<Stream>(), It.IsAny<StampType>(), It.IsAny<int[]?>(),
+                It.IsAny<(double, double)>(), It.IsAny<(double, double)?>(),
+                It.IsAny<AnnotationOrigin>(), It.IsAny<string?>(), It.IsAny<string?>(),
+                It.IsAny<double?>(), It.IsAny<string?>(), It.IsAny<double?>(),
+                It.IsAny<double?>(), It.IsAny<StampPlacement>(),
+                It.IsAny<byte[]?>(), It.IsAny<PredefinedStampType?>()))
+            .ThrowsAsync(new Exception("Unexpected"));
+
+        // Act & Assert
+        await Assert.ThrowsAsync<Exception>(() => _handler.Handle(command, CancellationToken.None));
+    }
+}