Closes the caller side of FLEX-WP-0015. FlexAuthHTTPAdapter reads the
audience-scoped projected ServiceAccount token from a file on every decision,
so hourly rotation needs no restart, and runtime configuration now requires
USER_ENGINE_FLEX_AUTH_TOKEN_FILE.
A missing, empty, or unreadable token file fails closed as a denial without
reaching flex-auth: OSError joins the caught set and an empty read raises.
Coverage proves all three unusable-token cases deny before any request is
made, and that neither the deny reason nor the decision repr carries the
token value.
Tenant-authority reads now identify user-engine as actor `user-engine` under
the protected tenant.read action, keeping tenant ids opaque and URL-encoded.
Contract: docs/flex-auth-caller-identity.md. Full suite: 148 tests, 3
provider-gated skips.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
