Toggle Between Discovered and OWL Ontology Schema Views in RDF Schema Explorer

## Problem

The RDF Schema Explorer currently only displays the schema discovered through sampling. For RDF databases that define an OWL ontology, users may want to see the formal ontology alongside or instead of the discovered schema. There is no way to compare what was discovered through sampling against what is explicitly defined in the ontology.

## Prerequisites

- OWL ontology fetching is implemented (separate backlog item).

## Proposed Solution

Add a toggle control in the Schema Explorer toolbar that allows users to switch between schema views:

- **Discovered** — The current behavior. Shows the schema inferred from sampling the data.
- **Ontology** — Shows the schema as defined by the OWL ontology.
- **Combined** — Merges both views into a single schema visualization, showing the union of discovered and ontology-defined types, properties, and relationships.

A toggle button group or radio group in the toolbar would work well for this. The labels could be something like "Discovered", "Ontology", and "Combined" (or "Merged"/"Hybrid").

## Considerations

- How to visually distinguish ontology-defined vs. discovered elements in the combined view (e.g., color coding, icons, or badges).
- How to handle conflicts where the discovered schema differs from the ontology (e.g., a property type discovered as `string` but defined as `xsd:dateTime` in the ontology).
- The toggle should only appear for RDF connections, or only when an OWL ontology is available.

## Acceptance Criteria

- [ ] A toggle control is available in the Schema Explorer toolbar for RDF connections with an available OWL ontology.
- [ ] Users can view the discovered schema, the OWL ontology schema, or a combined view.
- [ ] The combined view clearly indicates which elements come from the ontology vs. discovery.

Start at the RDF Schema Explorer toolbar and trace how the current sampling-based schema view is rendered. Review the separate OWL ontology fetching backlog item and define the view-state and combined-view behavior against the acceptance criteria; done means RDF connections with an available ontology can switch among Discovered, Ontology, and Combined views with clear element provenance.

Written by the indexing model from the issue text.