[EPIC] Controller/Backend Improvements

### Certification

- [x] I certify I am an Epic Owner for Kubeflow Notebooks 2.0 and expected to create planning-related issues.

### User Story

As an MLOps administrator managing Kubeflow Notebooks v2 in production, Joel needs the Workspace controller to handle edge cases gracefully, validate inputs thoroughly at admission time, and reconcile resources correctly across all supported Workspace and WorkspaceKind configurations — so that misconfigured resources are caught early, workspaces behave predictably without silent failures, and the CRD schemas are intuitive to configure with accurate documentation, examples, and clear error messages when something is wrong.

**Note**

This is planned as a community epic to continuously improve the robustness, correctness, and reliability of the Workspace controller. It will serve as a central point for tracking efforts related to reconciliation completeness, webhook validation coverage, CRD accuracy, and overall controller quality.

Start by breaking this epic into focused tasks for Workspace reconciliation, admission-time validation, CRD schema accuracy, error messages, and documentation or examples. Review the existing Workspace and WorkspaceKind behavior and identify a single reproducible gap with a clear expected outcome before proposing implementation work.

Written by the indexing model from the issue text.