fix(#134): Add generic get/set methods to CacheService

- Add get(key) method for arbitrary key-value cache access
- Add set(key, value, ttl) method with 5min default TTL
- Both methods handle Redis unavailability gracefully
- Proper error handling and logging
- Add 8 unit tests for new methods

Fixes AttributeError in /playground/validate-repo endpoint.

Author: DevanshuNEU

backend/services/cache.py

@@ -56,6 +56,47 @@ def _make_key(self, prefix: str, *args) -> str:
         hash_input = ":".join(parts)
         hash_val = hashlib.md5(hash_input.encode()).hexdigest()[:12]
         return f"{prefix}:{hash_val}"
+
+    def get(self, key: str) -> Optional[Dict]:
+        """
+        Get value by key (generic cache access).
+        
+        Used for arbitrary key-value caching like repo validation results.
+        """
+        if not self.redis:
+            return None
+        
+        try:
+            cached = self.redis.get(key)
+            if cached:
+                metrics.increment("cache_hits")
+                return json.loads(cached)
+            metrics.increment("cache_misses")
+        except Exception as e:
+            logger.error("Cache read error", operation="get", key=key[:50], error=str(e))
+            metrics.increment("cache_errors")
+        
+        return None
+
+    def set(self, key: str, value: Dict, ttl: int = 300) -> bool:
+        """
+        Set value by key (generic cache access).
+        
+        Used for arbitrary key-value caching like repo validation results.
+        Default TTL is 5 minutes.
+        
+        Returns True if successful, False otherwise.
+        """
+        if not self.redis:
+            return False
+        
+        try:
+            self.redis.setex(key, ttl, json.dumps(value))
+            return True
+        except Exception as e:
+            logger.error("Cache write error", operation="set", key=key[:50], error=str(e))
+            metrics.increment("cache_errors")
+            return False
 
     def get_search_results(self, query: str, repo_id: str) -> Optional[List[Dict]]:
         """Get cached search results"""

backend/tests/test_cache_service.py

@@ -0,0 +1,99 @@
+"""
+Tests for CacheService - specifically the generic get/set methods.
+"""
+import pytest
+from unittest.mock import MagicMock, patch
+import json
+
+
+class TestCacheServiceGenericMethods:
+    """Test the generic get() and set() methods added for #134."""
+    
+    @pytest.fixture
+    def mock_redis(self):
+        """Create a mock Redis client."""
+        return MagicMock()
+    
+    @pytest.fixture
+    def cache_service(self, mock_redis):
+        """Create CacheService with mocked Redis."""
+        with patch('services.cache.redis.Redis', return_value=mock_redis):
+            with patch('services.cache.redis.from_url', return_value=mock_redis):
+                mock_redis.ping.return_value = True
+                from services.cache import CacheService
+                service = CacheService()
+                service.redis = mock_redis
+                return service
+    
+    def test_get_returns_cached_value(self, cache_service, mock_redis):
+        """get() returns parsed JSON when key exists."""
+        test_data = {"valid": True, "repo_name": "flask"}
+        mock_redis.get.return_value = json.dumps(test_data).encode()
+        
+        result = cache_service.get("validate:https://github.com/pallets/flask")
+        
+        assert result == test_data
+        mock_redis.get.assert_called_once()
+    
+    def test_get_returns_none_when_key_missing(self, cache_service, mock_redis):
+        """get() returns None when key doesn't exist."""
+        mock_redis.get.return_value = None
+        
+        result = cache_service.get("validate:nonexistent")
+        
+        assert result is None
+    
+    def test_get_returns_none_when_redis_unavailable(self, cache_service):
+        """get() returns None when Redis is not available."""
+        cache_service.redis = None
+        
+        result = cache_service.get("any_key")
+        
+        assert result is None
+    
+    def test_get_handles_redis_error(self, cache_service, mock_redis):
+        """get() returns None and logs error on Redis exception."""
+        mock_redis.get.side_effect = Exception("Redis connection error")
+        
+        result = cache_service.get("validate:test")
+        
+        assert result is None
+    
+    def test_set_stores_value_with_ttl(self, cache_service, mock_redis):
+        """set() stores JSON value with TTL."""
+        test_data = {"valid": True, "can_index": True}
+        
+        result = cache_service.set("validate:test", test_data, ttl=300)
+        
+        assert result is True
+        mock_redis.setex.assert_called_once_with(
+            "validate:test",
+            300,
+            json.dumps(test_data)
+        )
+    
+    def test_set_uses_default_ttl(self, cache_service, mock_redis):
+        """set() uses 300s (5 min) as default TTL."""
+        test_data = {"valid": True}
+        
+        cache_service.set("validate:test", test_data)
+        
+        # Check that TTL is 300 (default)
+        call_args = mock_redis.setex.call_args
+        assert call_args[0][1] == 300  # TTL is second positional arg
+    
+    def test_set_returns_false_when_redis_unavailable(self, cache_service):
+        """set() returns False when Redis is not available."""
+        cache_service.redis = None
+        
+        result = cache_service.set("any_key", {"data": "value"})
+        
+        assert result is False
+    
+    def test_set_handles_redis_error(self, cache_service, mock_redis):
+        """set() returns False on Redis exception."""
+        mock_redis.setex.side_effect = Exception("Redis write error")
+        
+        result = cache_service.set("validate:test", {"data": "value"})
+        
+        assert result is False