fix(trust-graph): count discovery 5xx failures

[EfeDurmaz16]

Summary

• count discovery HTTP 5xx responses toward the identity-resolution circuit breaker
• reuse the same failure path for transport errors and upstream 5xx responses
• add regression coverage that the circuit opens after repeated 5xx responses and stops calling discovery

Verification

• pnpm --filter @fides/trust-graph exec vitest run test/trust-service.test.ts -t "opens the discovery circuit breaker"
• pnpm --filter @fides/trust-graph test
• pnpm --filter @fides/trust-graph lint
• pnpm build
• git diff --check