bug: ion-tab-button isn't focusable when no href

### Prerequisites

- [x] I have read the [Contributing Guidelines](https://github.com/ionic-team/ionic-framework/blob/main/docs/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 report this problem, without success.

### Ionic Framework Version

v8.x

### Current Behavior

`ion-tab-button` cannot be focused with the `tab` key when they do not have `href`

### Expected Behavior

Being able to select them with `tab`

### Steps to Reproduce

1. Use your `tab` until you select the `search` button.
2. You'll see you go direct to `search` without passing by other elements

### Code Reproduction URL

https://stackblitz.com/edit/2uoossd8?file=index.html

### Ionic Info

Ionic:

Ionic CLI : 5.4.16

Utility:

cordova-res : not installed
native-run : not installed

System:

NodeJS : v20.19.1
npm : 10.8.2
OS : Linux 5.0

### Additional Information
I'd be glad to do a PR.
I would add the following

```typescript
@HostBinding('tabIndex')
get tabIndex() {
return 1
}
```

Start at the ion-tab-button component entry point and reproduce the keyboard navigation behavior using the linked StackBlitz example. Check the no-href case first; done means ion-tab-button elements without href are reachable with the Tab key rather than being skipped.

索引モデルが issue の本文から書いたものです。