fix: guard validate_requested_model + expand test coverage (PR #649 follow-up)

- Wrap validate_requested_model in try/except so /model doesn't crash
  if validation itself fails (falls back to old accept+save behavior)
- Remove unnecessary sys.path.insert from both test files
- Expand test_model_validation.py: 4 → 23 tests covering normalize_provider,
  provider_model_ids, empty/whitespace/spaces rejection, OpenRouter format
  validation, custom endpoints, nous provider, provider aliases, unknown
  providers, fuzzy suggestions
- Expand test_cli_model_command.py: 2 → 5 tests adding known-model save,
  validation crash fallback, and /model with no argument

cli.py

@@ -2034,11 +2034,15 @@ class HermesCLI:
                 except Exception:
                     provider_for_validation = self.provider or self.requested_provider
 
-                validation = validate_requested_model(
-                    new_model,
-                    provider_for_validation,
-                    base_url=self.base_url,
-                )
+                try:
+                    validation = validate_requested_model(
+                        new_model,
+                        provider_for_validation,
+                        base_url=self.base_url,
+                    )
+                except Exception:
+                    # Validation itself failed — fall back to old behavior (accept + save)
+                    validation = {"accepted": True, "persist": True, "recognized": False, "message": None}
 
                 if not validation.get("accepted"):
                     print(f"(^_^) Warning: {validation.get('message')}")