fix(overlays): ensure overlayIndex is greater than any existing overlay in DOM

[amanmaurya92]

Issue number: resolves #31247

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What is the current behavior?

When a dynamically generated overlay (such as an ion-alert triggered by an ion-select with interface="alert") is opened inside an already active overlay (like an ion-modal), it can sometimes be rendered behind the active modal. This is caused by the internal lastOverlayIndex falling out of sync across different module boundaries (common in microfrontends or Angular Standalone Component architectures), causing the newer overlay to receive a lower or equal overlayIndex (and consequently a lower CSS z-index) than the active modal.

What is the new behavior?

• Modified prepareOverlay in core/src/utils/overlays.ts to actively query the DOM for all existing Ionic overlays.
• The function now calculates the true maximum overlayIndex currently present in the DOM.
• New overlays are guaranteed a mathematically higher overlayIndex by assigning the maximum between the module's lastOverlayIndex and the DOM's maxIndex + 1, ensuring the most recently presented overlay is always stacked on top.

Does this introduce a breaking change?

• Yes
• No

Other information

This fix natively relies on document.querySelectorAll to serve as a robust Single Source of Truth for the stacking order, which effectively bridges the gap in complex environments where @ionic/core modules might be duplicated or heavily code-split.

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