fix: Remove @internal annotation from snapshot_

[TheIronDev]

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This commit removes the @internal annotation from snapshot_, and fixes https://github.com/firebase/firebase-functions-test/issues/87.

snapshot_ is currently referenced here.

More context here

I'm happy to work through better alternatives too.

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Fixes https://github.com/firebase/firebase-functions-test/issues/79 🦕

[TheIronDev]

Is there an appropriate person I should ping for review? 🐼