fix(database): fix DatabasePagingSource pagination when using orderByChild

[demolaf]

Fixes #2000.

DatabasePagingSource was calling startAt(null, key) for all queries, but Firebase Database ignores the key parameter when the query uses orderByChild(), causing every subsequent page load to return the same first page of results and throwing IllegalStateException: The same value was passed as the nextKey in two sequential Pages.

The fix detects orderByChild() queries via the internal PathIndex and calls startAt(childValue, key) instead, storing both the child value and node key as the page cursor in the new DatabasePagingKey type.

Changes:

• DatabasePagingKey — made public (required for cross-package Kotlin access) with equals()/hashCode() (required by the Paging library for key diffing and state restoration)
• DatabasePagingSource — extends cursor fix to orderByValue() queries via ValueIndex in addition to orderByChild() via PathIndex; surfaces load errors to logcat via onErrorReturn
• FirebaseRecyclerPagingAdapter — replaces deprecated @OnLifecycleEvent with LifecycleEventObserver (removed in lifecycle 2.7.0)
• Adds instrumented test verifying no duplicate keys across pages when using orderByChild()

[gemini-code-assist]

Code Review

This pull request introduces a new DatabasePagingKey class to replace String as the paging key in DatabasePagingSource and DatabasePagingOptions, allowing the paging source to correctly handle queries ordered by child values without producing duplicate items across pages. It also adds integration tests to verify this behavior. The review feedback suggests making DatabasePagingKey public and implementing equals() and hashCode() to ensure compatibility with the Paging library, as well as simplifying numeric type handling in startAtChildValue by checking for Number instead of Double and Long separately.

[mikehardy]

looks like a perfect fix for the existing issue but may have a not-too-hard extension possibility to be even more general and fix the general case that 2000 was just a specific instance of