feat: added auth-gateway usage cache with single-flight 15s ttl fallback
- Added AbortSignal propagation and timeout-race handling for broker health, usage, refresh, and snapshot calls.
- Added single-flight usage-report caching with 15s TTL, per-caller abort races, and null-on-fail fallback.
- Expanded provider schemas and parse/build logic for cache metadata, headers, stop controls, and image/file content.
- Hardened auth flows by rejecting refresh sentinels and using timing-safe bearer-token comparisons.
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