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/variables.py

@@ -173,7 +173,7 @@ async def set_variables(client: N8nClient, variables: dict[str, str]) -> str:
                     # Variable exists, update it
                     await client.update_variable(key, value)
                     updated.append(key)
-                except:
+                except Exception:
                     # Variable doesn't exist, create it
                     await client.create_variable(key, value)
                     created.append(key)