fix(auth): prevent null hydration on pagehide/reload

[gorthitk]

Description

This PR addresses an issue where Firebase Auth unexpectedly hydrates to a null state (signing the user out) on page reload or when returning to a backgrounded/restored tab.

The Problem

During pagehide (e.g., page unload or reload), the browser can interrupt/abort active IndexedDB transactions. In WebKit/Safari, instead of throwing a catchable error when a transaction is aborted during unload, the getAll() request on the object store resolves with an empty array ([]).
Because [] is truthy, the SDK's polling loop (_poll()) processes it, compares it to the in-memory cache, assumes the auth keys were deleted in another tab, and propagates a null Auth state.

The Fix

1. Added pagehide and pageshow event listeners to IndexedDBLocalPersistence to set an isHiding flag, stop the polling timer, and close the IndexedDB connection during page transitions.
2. Guarded _openDb(), _withRetries(), and _poll() using the isHiding flag to prevent new database transactions from starting during pagehide, and discard any in-flight results.
3. Wrapped the polling body in a try/catch block to prevent unhandled promise rejections if the database transaction is aborted.

Closes #10041

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Latest commit: 2a91ddb

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[gemini-code-assist]

Code Review

This pull request introduces lifecycle event handling for pagehide and pageshow in IndexedDBLocalPersistence to gracefully close the database and stop polling when the page is hidden, and resume polling when the page is shown. Feedback was provided to optimize the pageshow event listener by adding an if (this.isHiding) guard to prevent redundant polling timer restarts on initial page load.

[macastelaz]

Thanks for putting this fix together!