Simplify image annotator component when allowed_shapes=["rect"]

#### Is your feature request related to a problem? Please describe

The addition of polygon annotation support slows down the process for box annotations when using the `ui.image_annotator` component.

#### Describe the solution you'd like

If only box annotation is allowed (`allowed_shapes=["rect"]` ) , it would be helpful to fall back to a combined Select/Rectangle tool. Requiring the extra clicks slows down the annotation process. I do understand that they are needed for polygon annotations.

#### Describe alternatives you've considered

Keyboard shortcuts (https://github.com/h2oai/wave/issues/1596) , which could further speed up the process but should not be seen as a replacement for this issue.

Start by locating the ui.image_annotator implementation and its handling of allowed_shapes=["rect"]. Check how rectangle and polygon tools are selected, then add coverage for the rect-only workflow. Done means box annotation avoids the extra tool-selection click while polygon annotation behavior remains unchanged.

Written by the indexing model from the issue text.