fix(tab-button): restore keyboard focus

[cpruijsen]

Enabled ion-tab-button controls without href can now receive keyboard focus.

Issue number: resolves #30648

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What is the current behavior?

Tab navigation skips enabled ion-tab-button controls without href because the native anchor has no tabindex.

What is the new behavior?

• core/src/components/tab-button/tab-button.tsx sets tabindex="0" on the native anchor when an enabled tab button has no href.
• Focus also works after removal of an existing href.
• Defined href values retain native anchor focus behavior. Disabled controls retain tabindex="-1".
• Spec tests cover missing href, href removal, and disabled state. The accessibility fixture also includes a tab button without href.

Does this introduce a breaking change?

• Yes
• No

Other information

The native <a> keeps the component's DOM structure and existing href behavior.

The missing-href and href-removal assertions fail against baseline source and pass with the change.
Accessibility tests passed across iOS/MD and LTR/RTL variants.

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