feat: add watermark (release 8.28)

[StevenRenaux]

Q	A
Gotenberg API version ?	8.28
Bug fix ?	no
New feature ?	yes
BC break ?	no

Description

• Add WatermarkSource enum (text, image, pdf)
• Add WatermarkTrait with 5 fields: watermarkSource, watermarkExpression, watermarkPages, watermarkOptions, watermarkFile
• Add WatermarkPdfBuilder (POST /forms/pdfengines/watermark) with full payload validation (requires Gotenberg >= 8.28)
• Compose WatermarkTrait into ChromiumPdfTrait, LibreOfficeTrait, MergePdfBuilder, SplitPdfBuilder
• Expose watermark() on GotenbergPdfInterface, GotenbergPdf, TraceableGotenbergPdf
• Add WatermarkTestCaseTrait + WatermarkPdfBuilderTest; extend existing builder tests
• Add 3 pre-existing NodeBuilder generics ignore entries to phpstan.dist.neon
• Regenerate API docs

[Jean-Beru]

Review by Claude (claude.ai/code).

This PR mirrors #250 (stamp) for watermark support. Same structure, same patterns. Three issues found -- two are shared with #250, one is specific to this PR.

[Jean-Beru]

Missing service registration and bundle wiring -- Review by Claude (claude.ai/code)

Compared with #250 (stamp), this PR is missing three key files:

1. config/builder_pdf.php -- no service definition for .sensiolabs_gotenberg.pdf_builder.watermark. Without this, the DI container won't know about WatermarkPdfBuilder.

2. src/SensiolabsGotenbergBundle.php -- WatermarkPdfBuilder::class is not registered via $extension->registerBuilder().

3. src/DependencyInjection/SensiolabsGotenbergExtension.php -- the watermark key is not added to the phpdoc type for the pdf config array.

Without these, calling $gotenberg->watermark() will fail at runtime because the builder won't be resolved from the container.

[Jean-Beru]

Same as the PR about stamps. It's a BC break. Maybe we can skip this change and deprecate this interface if not useful.

[StevenRenaux]

Depending of what you decide with @Neirda24