fix: preserve keepSynced state across Query.get

[mvanhorn]

Have the internal SyncTree.keepSynced operation report whether the requested transition actually added a registration, while retaining idempotent behavior for ordinary public keepSynced calls. Calling keepSynced(true) and get() on the same Realtime Database query can corrupt the SDK's internal keep-synced bookkeeping after the server response is applied.

With persistence disabled and no cached data, call keepSynced(true), await get(), call keepSynced(true) again for the same path, and await another get(); both tasks return the server value without a duplicate-listen assertion; Repeat the second synchronization and read through a newly created DatabaseReference for the same path to verify deduplication follows QuerySpec identity rather than Java object identity.

Fixes #8433

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