test(server-utils): Rewrite firebase orchestrion tests as direct unit tests

Drop the SDK-setup harness (installTestAsyncContextStrategy + setupOnce +
tracingChannel round-trip) and call the exported functions directly. Tests
that only existed to exercise the channel wiring are removed: the deleteDoc
error lifecycle is generic bind behavior already covered in
tracing-channel.test.ts, and the missing-reference / parent-selection cases
live in the instrumentFirebase closure (covered by the orchestrion e2e).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: nicohrubec