feat: config structure validation — detect malformed YAML at startup (#5426)

Add validate_config_structure() that catches common config.yaml mistakes:
- custom_providers as dict instead of list (missing '-' in YAML)
- fallback_model accidentally nested inside another section
- custom_providers entries missing required fields (name, base_url)
- Missing model section when custom_providers is configured
- Root-level keys that look like misplaced custom_providers fields

Surface these diagnostics at three levels:
1. Startup: print_config_warnings() runs at CLI and gateway module load,
   so users see issues before hitting cryptic errors
2. Error time: 'Unknown provider' errors in auth.py and model_switch.py
   now include config diagnostics with fix suggestions
3. Doctor: 'hermes doctor' shows a Config Structure section with all
   issues and fix hints

Also adds a warning log in runtime_provider.py when custom_providers
is a dict (previously returned None silently).

Motivated by a Discord user who had malformed custom_providers YAML
and got only 'Unknown Provider' with no guidance on what was wrong.

17 new tests covering all validation paths.

hermes_cli/doctor.py

@@ -318,6 +318,25 @@ def run_doctor(args):
         except Exception:
             pass
 
+        # Validate config structure (catches malformed custom_providers, etc.)
+        try:
+            from hermes_cli.config import validate_config_structure
+            config_issues = validate_config_structure()
+            if config_issues:
+                print()
+                print(color("◆ Config Structure", Colors.CYAN, Colors.BOLD))
+                for ci in config_issues:
+                    if ci.severity == "error":
+                        check_fail(ci.message)
+                    else:
+                        check_warn(ci.message)
+                    # Show the hint indented
+                    for hint_line in ci.hint.splitlines():
+                        check_info(hint_line)
+                    issues.append(ci.message)
+        except Exception:
+            pass
+
     # =========================================================================
     # Check: Auth providers
     # =========================================================================