PDFCLOUD-5554 Add client methods to watermark documents

[datalogics-cgreen]

PDFCLOUD-5554

Adds a new Watermark PDF client helper and supporting payload/type definitions.

Introduces comprehensive unit and live test coverage for the watermark endpoint.

[datalogics-cgreen]

@datalogics-kam If you like how text and image watermarks have been split up, I can squash the earlier version (commits).

[datalogics-kam]

Let's avoid squashing for now, it makes it hard to reason about the changes. I think for human-originated stuff and small changes it's ok, but let's keep the coding agent history.

[datalogics-kam]

1. Please remove default-merging validation; if anything, that kind of stuff belongs on the payload. If there are problems with default values going to the server, let's talk.
2. Let's give the user one color with two possible types, and the default is RGB black.
3. Codex complained about changing the name of PdfCmykColor, but I'm glad we fixed that inconsistency.

Codex findings from PR Review skill:

1. [P2] Live coverage gap for split watermark endpoints (missing async image path)

   • watermark_pdf_with_image has live sync coverage but no matching live async success case, so transport parity is incomplete for the new split API surface.
   • Evidence: /Users/kam/src/dl/pdfrest-python/tests/live/test_live_watermark_pdf.py:76 covers sync image; no corresponding async image success test exists in that module.
   • Risk: async-only regressions in image watermark request/response behavior could ship undetected.

2. [P2] Missing request-customization/timeout coverage for image watermark helper

   • The suite verifies extra_query/extra_headers/extra_body/timeout only for watermark_pdf_with_text, not for watermark_pdf_with_image.
   • Evidence: customization tests are text-only at /Users/kam/src/dl/pdfrest-python/tests/test_watermark_pdf.py:149 and /Users/kam/src/dl/pdfrest-python/tests/test_watermark_pdf.py:435; no image equivalent present.
   • Risk: plumbing regressions specific to the image helper (headers/query/body merge or timeout propagation) may slip through.

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[datalogics-kam]

Please address the changes requested. I will make modifications to AGENTS.md to cause Codex to follow the conventions in the future.

[datalogics-cgreen]

01e398d adds a BeforeValidator intended to set the correct color parameter on the payload.

45471b1 and 2284c55 clean up further by setting the default text color on the client side and ensuring it's formatted properly in the request payload, respectively.

[datalogics-kam]

The text_color distribution to pdfRest fields is even more clever than I thought...approved.