🛡️ fix: Improve Error Handling and Null Safety in SSE Event Processing (#10751)

* 🔧 fix: Handle null content parts in message processing

- Added checks to filter out null content parts in various message handling functions, ensuring robustness against undefined values.
- Updated the `extractMessageContent`, `useContentHandler`, `useEventHandlers`, and `useStepHandler` hooks to prevent errors caused by null parts.
- Enhanced the `getAllContentText` utility to only include valid content types, improving overall message integrity.

* 🔧 fix: Enhance error handling in event and SSE handlers

- Wrapped critical sections in try-catch blocks within `useEventHandlers` and `useSSE` hooks to improve error management and prevent application crashes.
- Added console error logging for better debugging and tracking of issues during message processing and conversation aborting.
- Ensured that UI states like `setIsSubmitting` and `setShowStopButton` are correctly updated in case of errors, maintaining a consistent user experience.

* 🔧 fix: Filter out null and empty content in message export

- Enhanced the `useExportConversation` hook to filter out null content parts and empty strings during message processing, ensuring only valid content is included in the export.
- This change improves the integrity of exported conversations by preventing unnecessary empty entries in the output.

client/src/hooks/SSE/useStepHandler.ts

@@ -313,6 +313,10 @@ export default function useStepHandler({
             ? messageDelta.delta.content[0]
             : messageDelta.delta.content;
 
+          if (contentPart == null) {
+            return;
+          }
+
           const currentIndex = calculateContentIndex(
             runStep.index,
             initialContent,
@@ -345,6 +349,10 @@ export default function useStepHandler({
             ? reasoningDelta.delta.content[0]
             : reasoningDelta.delta.content;
 
+          if (contentPart == null) {
+            return;
+          }
+
           const currentIndex = calculateContentIndex(
             runStep.index,
             initialContent,