fix: Prevent duplicate task creation by refactoring webview message handling

This commit resolves an issue where tasks were being created multiple times when initiated from the webview. The root cause was in the ClineProvider's webview message handling implementation.

Removed WebviewMessageHandlers class with its large switch statement
Replaced indirect message handler setup with direct function in resolveWebviewView
Removed redundant setWebviewMessageListener method
Simplified message handling flow using command pattern consistently
Updated tests to properly mock webview message handling
Each webview message is now processed exactly once, preventing duplicate task creation and improving stability.

Author: emshvac

Progress-SettingsManager-Refactoring-Plan.md

@@ -328,12 +328,20 @@ Despite the successful completion of Phase 3, the ClineProvider.ts file is still
 
 Phase 4 will focus on completing the transition to the new architecture by removing legacy code and simplifying the tests.
 
-#### Step 1: Remove Switch Statement (Planned)
+#### Step 1: Remove Switch Statement ✅ (Completed)
 
-- [ ] Remove the large switch statement in setWebviewMessageListener
-- [ ] Ensure all message types are handled by command handlers
-- [ ] Update any remaining code that relies on the switch statement
-- [ ] Test thoroughly to ensure no functionality is lost
+- [x] Remove the large switch statement in setWebviewMessageListener
+- [x] Ensure all message types are handled by command handlers
+- [x] Update any remaining code that relies on the switch statement
+- [x] Test thoroughly to ensure no functionality is lost
+
+##### Issues and Fixes
+
+1. **Multiple Task Creation Issue**
+    - **Issue**: After removing the switch statement, tasks were being created multiple times.
+    - **Root Cause**: The `generateSystemPrompt` function was still defined at the end of the `setWebviewMessageListener` method but wasn't being used anywhere. This leftover function was causing issues with message handling.
+    - **Fix**: Removed the unused `generateSystemPrompt` function from the `setWebviewMessageListener` method.
+    - **Result**: Tasks are now created only once as expected.
 
 #### Step 2: Simplify Test Setup (Planned)