refactor: Extract image validation to provider base class

Consolidates duplicated image validation logic from individual providers
into a reusable base class method. This improves maintainability and
ensures consistent validation across all providers.

- Added validate_image() method to ModelProvider base class
- Supports both file paths and data URLs
- Validates image format, size, and MIME types
- Added DEFAULT_MAX_IMAGE_SIZE_MB class constant (20MB)
- Refactored Gemini and OpenAI providers to use base validation
- Added comprehensive test suite with 19 tests
- Used minimal mocking approach with concrete test provider class

providers/gemini.py

@@ -2,7 +2,6 @@
 
 import base64
 import logging
-import os
 import time
 from typing import Optional
 
@@ -440,28 +439,22 @@ class GeminiModelProvider(ModelProvider):
     def _process_image(self, image_path: str) -> Optional[dict]:
         """Process an image for Gemini API."""
         try:
-            if image_path.startswith("data:image/"):
-                # Handle data URL: data:image/png;base64,iVBORw0...
-                header, data = image_path.split(",", 1)
-                mime_type = header.split(";")[0].split(":")[1]
+            # Use base class validation
+            image_bytes, mime_type = self.validate_image(image_path)
+
+            # For data URLs, extract the base64 data directly
+            if image_path.startswith("data:"):
+                # Extract base64 data from data URL
+                _, data = image_path.split(",", 1)
                 return {"inline_data": {"mime_type": mime_type, "data": data}}
             else:
-                # Handle file path
-                from utils.file_types import get_image_mime_type
-
-                if not os.path.exists(image_path):
-                    logger.warning(f"Image file not found: {image_path}")
-                    return None
-
-                # Detect MIME type from file extension using centralized mappings
-                ext = os.path.splitext(image_path)[1].lower()
-                mime_type = get_image_mime_type(ext)
-
-                # Read and encode the image
-                with open(image_path, "rb") as f:
-                    image_data = base64.b64encode(f.read()).decode()
-
+                # For file paths, encode the bytes
+                image_data = base64.b64encode(image_bytes).decode()
                 return {"inline_data": {"mime_type": mime_type, "data": image_data}}
+
+        except ValueError as e:
+            logger.warning(str(e))
+            return None
         except Exception as e:
             logger.error(f"Error processing image {image_path}: {e}")
             return None