fix(#892): Gateway config validation and fallback fixes

Config validator and fallback fixes:
- Validate required keys (OPENROUTER_API_KEY, API_SERVER_KEY)
- Fix idle_minutes validation (>0 required)
- Fix Discord skill limit (reduce to 95 max)
- Validate provider configs
- Apply sensible defaults

Resolves #892

gateway/config_validator.py

@@ -0,0 +1,224 @@
+"""
+Gateway Config Validator & Fallback Fix — #892.
+
+Validates gateway configuration and provides sensible defaults
+for missing keys to prevent fallback chain breaks.
+"""
+
+import logging
+import os
+from typing import Dict, Any, List, Optional
+from dataclasses import dataclass, field
+
+logger = logging.getLogger(__name__)
+
+
+@dataclass
+class ConfigIssue:
+    """A configuration issue found during validation."""
+    key: str
+    severity: str  # error, warning, info
+    message: str
+    fix: str
+
+
+@dataclass
+class ConfigValidation:
+    """Result of config validation."""
+    valid: bool
+    issues: List[ConfigIssue] = field(default_factory=list)
+    warnings: int = 0
+    errors: int = 0
+
+
+# Required keys and their defaults
+REQUIRED_KEYS = {
+    "OPENROUTER_API_KEY": {
+        "required": False,
+        "default": "",
+        "severity": "warning",
+        "message": "OPENROUTER_API_KEY not set - fallback chain may break",
+        "fix": "Set OPENROUTER_API_KEY in .env for OpenRouter provider",
+    },
+    "API_SERVER_KEY": {
+        "required": False,
+        "default": "",
+        "severity": "warning",
+        "message": "API_SERVER_KEY not configured",
+        "fix": "Set API_SERVER_KEY in .env for API server auth",
+    },
+    "GITEA_TOKEN": {
+        "required": False,
+        "default": "",
+        "severity": "info",
+        "message": "GITEA_TOKEN not set - Gitea features disabled",
+        "fix": "Set GITEA_TOKEN in .env for Gitea integration",
+    },
+}
+
+# Config validation rules
+VALIDATION_RULES = [
+    {
+        "key": "idle_minutes",
+        "validate": lambda v: isinstance(v, (int, float)) and v > 0,
+        "message": "Invalid idle_minutes={v} - must be > 0",
+        "fix": "Set idle_minutes to positive integer (default: 30)",
+    },
+    {
+        "key": "max_skills_discord",
+        "validate": lambda v: isinstance(v, int) and v <= 100,
+        "message": "Discord slash command limit reached ({v}/100) - skills not registered",
+        "fix": "Reduce skills or paginate registration",
+    },
+]
+
+
+def validate_config(config: Dict[str, Any]) -> ConfigValidation:
+    """
+    Validate gateway configuration.
+    
+    Args:
+        config: Configuration dictionary
+        
+    Returns:
+        ConfigValidation with issues found
+    """
+    issues = []
+    
+    # Check required keys
+    for key, spec in REQUIRED_KEYS.items():
+        value = config.get(key) or os.environ.get(key) or spec["default"]
+        if spec["required"] and not value:
+            issues.append(ConfigIssue(
+                key=key,
+                severity=spec["severity"],
+                message=spec["message"],
+                fix=spec["fix"],
+            ))
+        elif not value and spec["severity"] != "error":
+            issues.append(ConfigIssue(
+                key=key,
+                severity=spec["severity"],
+                message=spec["message"],
+                fix=spec["fix"],
+            ))
+    
+    # Check validation rules
+    for rule in VALIDATION_RULES:
+        value = config.get(rule["key"])
+        if value is not None:
+            if not rule["validate"](value):
+                issues.append(ConfigIssue(
+                    key=rule["key"],
+                    severity="error",
+                    message=rule["message"].format(v=value),
+                    fix=rule["fix"],
+                ))
+    
+    errors = sum(1 for i in issues if i.severity == "error")
+    warnings = sum(1 for i in issues if i.severity == "warning")
+    
+    return ConfigValidation(
+        valid=errors == 0,
+        issues=issues,
+        warnings=warnings,
+        errors=errors,
+    )
+
+
+def apply_defaults(config: Dict[str, Any]) -> Dict[str, Any]:
+    """
+    Apply default values for missing config keys.
+    
+    Args:
+        config: Configuration dictionary
+        
+    Returns:
+        Config with defaults applied
+    """
+    result = dict(config)
+    
+    for key, spec in REQUIRED_KEYS.items():
+        if key not in result or not result[key]:
+            default = os.environ.get(key) or spec["default"]
+            if default:
+                result[key] = default
+                logger.debug("Applied default for %s", key)
+    
+    # Apply validation defaults
+    if "idle_minutes" not in result or not result["idle_minutes"] or result["idle_minutes"] <= 0:
+        result["idle_minutes"] = 30
+        logger.debug("Applied default idle_minutes=30")
+    
+    return result
+
+
+def fix_discord_skill_limit(skills: List[str], max_skills: int = 95) -> List[str]:
+    """
+    Fix Discord slash command limit by reducing skills.
+    
+    Args:
+        skills: List of skill names
+        max_skills: Maximum skills to register (default 95, leaving room for built-ins)
+        
+    Returns:
+        Reduced skill list
+    """
+    if len(skills) <= max_skills:
+        return skills
+    
+    logger.warning(
+        "Discord skill limit: %d skills exceeds %d limit, truncating",
+        len(skills), max_skills
+    )
+    
+    # Keep first max_skills (alphabetical priority)
+    return sorted(skills)[:max_skills]
+
+
+def validate_provider_config(provider: str, config: Dict[str, Any]) -> ConfigIssue:
+    """
+    Validate provider-specific configuration.
+    
+    Args:
+        provider: Provider name
+        config: Provider config
+        
+    Returns:
+        ConfigIssue if invalid, None if valid
+    """
+    if provider == "local-llama.cpp":
+        # Check if llama.cpp is configured
+        if not config.get("model_path") and not config.get("base_url"):
+            return ConfigIssue(
+                key=f"provider.{provider}",
+                severity="warning",
+                message=f"{provider} provider not configured - fallback fails",
+                fix=f"Configure {provider} model_path or base_url, or remove from provider list",
+            )
+    
+    return None
+
+
+def format_validation_report(validation: ConfigValidation) -> str:
+    """Format validation results as a report."""
+    lines = [
+        "=" * 50,
+        "GATEWAY CONFIG VALIDATION",
+        "=" * 50,
+        "",
+        f"Status: {'VALID' if validation.valid else 'INVALID'}",
+        f"Errors: {validation.errors}",
+        f"Warnings: {validation.warnings}",
+        "",
+    ]
+    
+    if validation.issues:
+        lines.append("Issues:")
+        for issue in validation.issues:
+            icon = "❌" if issue.severity == "error" else "⚠️" if issue.severity == "warning" else "ℹ️"
+            lines.append(f"  {icon} [{issue.key}] {issue.message}")
+            lines.append(f"     Fix: {issue.fix}")
+            lines.append("")
+    
+    return "\n".join(lines)