ref(service): Classify service errors by kind

Add service-level error kinds so API handlers can map failures without matching every backend-specific variant. Keep client metadata errors distinct from internal metadata failures and classify transient backend failures as retryable/service-unavailable paths.

Co-Authored-By: OpenAI Codex <noreply@openai.com>

Author: lcian

Cargo.lock

@@ -34,6 +34,7 @@ stresstest = { path = "stresstest" }
 anyhow = "1.0.69"
 async-trait = "0.1.88"
 bytes = "1.11.1"
+derive-error-kind = "0.1.0"
 futures = "0.3.31"
 futures-util = "0.3.31"
 http = "1.3.1"