Enhance error handling and validation in agents and routes

- Added logging for empty responses in GeminiAgent and OpenRouterAgent to track potential session context issues.
- Refactored settings file path usage in OpenRouterAgent to use a constant for better maintainability.
- Improved error handling in SessionRoutes to log generator failures with detailed context.
- Implemented JSON parsing error handling in SettingsRoutes to manage corrupted settings files gracefully.
- Added validation for CLAUDE_MEM_OPENROUTER_MAX_CONTEXT_MESSAGES, CLAUDE_MEM_OPENROUTER_MAX_TOKENS, and CLAUDE_MEM_OPENROUTER_SITE_URL in SettingsRoutes to ensure valid configuration.

src/services/worker/GeminiAgent.ts

@@ -170,6 +170,11 @@ export class GeminiAgent {
 
         // Process response (no original timestamp for init - not from queue)
         await this.processGeminiResponse(session, initResponse.content, worker, tokensUsed, null);
+      } else {
+        logger.warn('SDK', 'Empty Gemini init response - session may lack context', {
+          sessionId: session.sessionDbId,
+          model
+        });
       }
 
       // Process pending messages