Dock: Custom tooltip content for dock items

### Description of the new feature / enhancement

Allow dock items to provide custom tooltip content using `IContent[]`.

This should be separate from an actionable custom flyout. The goal is to support informational hover content only, shown on pointer hover rather than on click.

This would let extensions provide richer contextual details for dock items.

### Scenario when this would be used?

UX

### Supporting information

_No response_

Start by locating the dock-item extension API and the existing custom flyout behavior. Trace how tooltip content is represented and determine where pointer-hover and click interactions are handled. Done means dock items can expose IContent[] for informational hover content without turning it into an actionable custom flyout.

Written by the indexing model from the issue text.