Add OKLab and Oklch image utilities and nodes, refactor color space nodes

[JPPhoto]

Summary

Added shared OKLab/OKLCH and related color-conversion utilities, a new unsharp_mask_oklab image node, and a new img_hue_adjust_oklch image node. The PR also consolidates color-conversion logic into the shared backend module, exports the public helpers through invokeai.invocation_api for custom nodes, standardizes public hue contracts to degrees for polar/hue-based spaces, and regenerates frontend API types/schema.

This PR also fixes an existing Oklch blend bug in composition-nodes: main was treating Oklch hue inconsistently and wrapping it like a unit-range value in some blend paths. The refactor standardizes Oklch hue to degrees throughout the shared/public conversion path and updates blend wrapping accordingly, so Oklch blend results may differ slightly from main where the old behavior was incorrect.

Related Issues / Discussions

QA Instructions

• Run pytest tests/backend/image_util/test_color_conversion.py tests/app/invocations/test_image.py -q

Merge Plan

Checklist

• The PR has a short but descriptive title, suitable for a changelog
• Tests added / updated (if applicable)
• ❗Changes to a redux slice have a corresponding migration
• Documentation added / updated (if applicable)
• Updated What's New copy (if doing a release after this PR)

[JPPhoto]

@dwringer Mind playing with composition nodes here to see if they do what they used to? I made a lot of changes to their internals and fixed what I believe was a bug while I was at it.