Refactor categorical space dimensions

[knobel-dk]

Summary

This refactors the categorical space dimension implementation by introducing a private _BaseCategorical helper that contains behavior shared by Categorical and Task.

Changes

• Moves shared categorical setup, prior handling, equality, repr helpers, bounds, transformed bounds, containment, distance, and prior-based sampling into _BaseCategorical.
• Keeps Categorical and Task as the public classes, with Task retaining only the active-task-specific behavior.
• Removes the duplicate Task.sample implementation so Task uses the inherited Dimension.sample behavior just like the other dimensions.

Validation

• Ran python3 -m py_compile ProcessOptimizer/space/space.py.
• Ran git diff --check.
• Ran a focused isolated smoke check for Categorical and Task behavior with uv.