feat(error-handling): add network error differentiation, retry, and REST API detection (C.5)

WordPress client improvements:
- Add ConnectionError exception for network-level failures
- Differentiate DNS, SSL, timeout, and connection refused errors
- Add retry with exponential backoff for transient errors (502, 503, 504, 429)
- Auth/config errors are never retried
- Add request timeout (30s default)

Health check improvements:
- Detect REST API disabled (403/404 on /wp-json)
- Classify errors by type: dns_failure, ssl_error, timeout, connection_refused

Bare except cleanup:
- Replaced all bare except: with except Exception: across 12 files

Tests: 303 passed (13 new tests for error handling)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

plugins/n8n/handlers/system.py

@@ -112,7 +112,7 @@ async def get_instance_info(client: N8nClient) -> str:
         try:
             health = await client.health_check()
             info["health"] = health
-        except:
+        except Exception:
             pass
 
         # Get current user to verify connectivity
@@ -123,7 +123,7 @@ async def get_instance_info(client: N8nClient) -> str:
                 "email": user.get("email"),
                 "role": user.get("role"),
             }
-        except:
+        except Exception:
             pass
 
         info["instance_url"] = client.site_url