fix(rust): wire up retry logic, URL validation, SSE reconnection, document reply

- #4 HIGH: request_with_retry wraps all API calls with exponential backoff
  on 429/500/502/503/504 (uses stored max_retries, default 3)
- #13 MEDIUM: base URL validated for http(s) scheme in HaiClient::new()
- #14 MEDIUM: on_benchmark_job now reconnects with exponential backoff
  (max 10 attempts, stops on explicit "disconnect" event)
- #17 MEDIUM: document why Rust uses server-side /email/reply endpoint;
  add reply endpoint to fixtures/contract_endpoints.json
- All tests passing (config validation from prior commit + new tests)

https://claude.ai/code/session_01MqxtVDf8nA4wPaPZD56zCM

Author: claude

fixtures/contract_endpoints.json

@@ -14,5 +14,11 @@
     "method": "POST",
     "path": "/api/v1/agents/jobs/{job_id}/response",
     "auth_required": true
+  },
+  "reply": {
+    "method": "POST",
+    "path": "/api/agents/{agent_id}/email/reply",
+    "auth_required": true,
+    "note": "Server-side reply endpoint. Rust SDK uses this directly; Python/Node/Go currently fetch the original client-side and compose locally. Other SDKs should migrate to this endpoint for consistency."
   }
 }