User-configured pass-through endpoints with ``auth: false`` are
explicitly unauthenticated — the builder short-circuits at
check_api_key_for_custom_headers_or_pass_through_endpoints and returns
a fresh empty UserAPIKeyAuth() without an api_key, user_id, or role.
Pre-refactor, that empty token never reached common_checks. After the
centralization, it does — and common_checks rejects it as admin-only,
breaking every Langfuse / custom unauthenticated pass-through.
This is the same regression class as the public-routes one: a
builder fast-path whose return value cannot survive common_checks.
Honor the same contract here — when the matched endpoint config has
auth != True, skip the centralized gate. auth=True endpoints still
run the full gate (covered by a companion test).
No security regression: ``auth: false`` is the operator's explicit
opt-out from LiteLLM auth on this path. The original commit closed
seven authenticated bypasses; this exemption applies only to a path
the operator has already declared unauthenticated.
7a91d80f9a