Refactor: Extract image processing logic from DocumentAssembler into ImageHelper

[Copilot]

Addresses review feedback from PR #165 to extract ~430 lines of image-specific functionality from DocumentAssembler.cs into a dedicated helper class.

Changes

Created ImageHelper.cs

• Static internal class handling all image operations:
  • Image part management (AddImagePart)
  • XML element generation (CreateImageElement)
  • Dimension calculations with unit conversions (px, pt, cm, mm, in, emu)
  • Format detection fallbacks (PNG, JPEG, GIF header parsing)
  • Alignment/justification processing

Updated DocumentAssembler.cs

• Replaced 12 private static methods with ImageHelper.* calls
• Removed image-specific imports (SkiaSharp, System.Globalization)
• Reduced from 1404 to 969 lines

Example

// Before
var imagePart = AddImagePart(owningPart);
var docPrId = GetNextDocPrId(owningPart);
var imageElement = CreateImageElement(relationshipId, docPrId, widthEmu, heightEmu, justification);

// After  
var imagePart = ImageHelper.AddImagePart(owningPart);
var docPrId = ImageHelper.GetNextDocPrId(owningPart);
var imageElement = ImageHelper.CreateImageElement(relationshipId, docPrId, widthEmu, heightEmu, justification);

All 789 tests pass without modification.

Original prompt

can you address https://github.com/Codeuctivity/OpenXmlPowerTools/pull/165/files#r2498803135

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[Copilot]

Pull Request Overview

This PR extracts approximately 430 lines of image-specific functionality from DocumentAssembler.cs into a dedicated ImageHelper.cs class, improving code organization and maintainability. The refactoring addresses review feedback from PR #165.

Key changes:

• Created new ImageHelper static class with 12 image-related methods
• Added support for <Image> template element with alignment and dimension attributes
• Updated DocumentAssembler to use ImageHelper methods and pass owningPart parameter through the transformation pipeline

Reviewed Changes

Copilot reviewed 10 out of 12 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
OpenXmlPowerTools/DocumentAssembler/ImageHelper.cs	New helper class containing all image processing logic including dimension calculations, format detection, and XML element generation
OpenXmlPowerTools/DocumentAssembler/DocumentAssembler.cs	Refactored to delegate image operations to ImageHelper and added Image element processing
OpenXmlPowerTools/DocumentAssembler/PA.cs	Added Image-related attribute name constants
OpenXmlPowerTools/HtmlToWmlConverterCore.cs	Enhanced font size handling with validation and rounding
OpenXmlPowerTools.Tests/DocumentAssemblerTests.cs	Added comprehensive test coverage for image functionality
OpenXmlPowerToolsExamples/DocumentAssembler04/*	New example demonstrating Image template element usage
DOCUMENT_ASSEMBLER.md	New documentation file for DocumentAssembler features

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[Copilot]

Generic catch clause.

Suggested change

	catch
	{
	// ignore and fall back to header-based detection
	catch (Exception ex)
	{
	errorMessage = $"Image: Failed to decode image using SKBitmap. Exception: {ex.Message}";
	// fall back to header-based detection