Add focused serve-side tests for the gates around peer requests. GetGame now has
coverage for the non-catalog, active-operation, and missing-sentinel cases that
should return GameNotFound instead of exposing local files.
The full-file and chunk handlers both depend on the same transfer gate before
touching the QUIC send stream. Extract that gate into a small helper and test
the same cases there, plus the existing local-path exclusion, so both dispatch
paths stay aligned without adding fake QUIC stream plumbing.
Test Plan:
- git diff --check
- just fmt
- just clippy
- just test
Follow-up-Plan: FOLLOW_UP_2.md
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