fix(segment): expose tab position metadata

[Mahesh5R]

Issue number: resolves #31282

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What is the current behavior?

Screen readers may not correctly announce the position and total count of
ion-segment-button tabs.

The native elements with role="tab" are rendered within each segment
button's Shadow DOM, so assistive technologies may not reliably infer the
position of each tab within the segment.

What is the new behavior?

ion-segment now calculates the position and total number of its segment
buttons and provides that information to each ion-segment-button.

The native button with role="tab" now exposes:

• aria-posinset
• aria-setsize

The values are recalculated when segment buttons are dynamically added or
removed.

Does this introduce a breaking change?

• Yes
• No

Other information

Added regression tests covering:

• ARIA position/count for initially rendered segment buttons
• dynamically adding a segment button
• dynamically removing a segment button and recalculating remaining positions

The existing Shadow DOM structure is unchanged.

Validation completed successfully:

• targeted segment tests
• TypeScript lint
• Sass lint
• Prettier
• production build

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