feat: toolbar, access toolbar background element via css

### Prerequisites

- [X] I have read the [Contributing Guidelines](https://github.com/ionic-team/ionic-framework/blob/main/.github/CONTRIBUTING.md#creating-an-issue).
- [X] I agree to follow the [Code of Conduct](https://ionicframework.com/code-of-conduct).
- [X] I have searched for [existing issues](https://github.com/ionic-team/ionic-framework/issues) that already include this feature request, without success.

### Describe the Feature Request

I need to apply a transition on the background of ion-toolbar, and there is currently no way to access the toolbar-background class.

### Describe the Use Case

be able to apply styles as transitions in the background-color

### Describe Preferred Solution

Adding a shadow part.

### Describe Alternatives

_No response_

### Related Code

_No response_

### Additional Information

_No response_

Start at the ion-toolbar component implementation and inspect how the toolbar-background element is rendered and whether existing shadow parts are exposed. Check the component’s related tests or documentation if available. Done means consumers can target the background element with a shadow part and apply background transitions without changing internal selectors.

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