Enhance DocumentAssembler image handling with sizing, alignment, fallbacks, and comprehensive tests

[mapo80]

• Extend < Image > metadata support in DocumentAssembler to honor Align, Width, Height, MaxWidth, and MaxHeight, preserving aspect ratios and mapping paragraph justification (w:jc) accordingly.
• Parse CSS-like length units (px, pt, cm, mm, in, emu), apply both max constraints safely, and guard against invalid/zero values with contextual template errors.
• Add resilience when SkiaSharp cannot decode a payload by falling back to PNG/JPEG/GIF header inspection to recover intrinsic dimensions, reducing failures on truncated or malformed data.
  Introduce helper utilities to compute unique image IDs, format justification values, and reuse paragraph extraction logic in tests.
• Expand DocumentAssemblerTests with scenarios covering width/height scaling, combined max constraints, alignment variants, optional placeholders, invalid metadata, and truncated GIFs, achieving >90% coverage of the new logic.
• Ship a new OpenXmlPowerToolsExamples/DocumentAssembler04 sample (template, sample data, SkiaSharp image generator) and rename DOCUMENT_ASSEMBLER.md to document image usage and metadata options.
• Update .gitignore to exclude generated ExampleOutput* folders so sample runs stay clean.

Tests

• dotnet test OpenXmlPowerTools.Tests/OpenXmlPowerTools.Tests.csproj -p:TargetFrameworks=net8.0 -nologo -clp:ErrorsOnly
• dotnet build OpenXmlPowerToolsExamples/DocumentAssembler04/DocumentAssembler04.csproj -p:TargetFrameworks=net8.0 -nologo -clp:ErrorsOnly

[github-actions]

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I have read the CLA Document and I hereby sign the CLA

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1 out of 3 committers have signed the CLA.
✅ (mapo80)[https://github.com/mapo80]
❌ @Copilot
❌ @politom-linksmt
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[stesee]

Can SkiaSharp ever fail to parse a valid image? If not, I recommend failing fast and returning an error here instead of trying to render invalid images — that keeps failures visible and avoids silent corruption.