fix: improve error handling and logging in summary and transcript processing

- Enhanced error handling in summary generation by using fallback messages for missing assistant messages.
- Updated the `buildSummaryPrompt` function to streamline the retrieval of the last assistant message.
- Improved the `extractLastMessage` function to log errors when transcript files are missing or empty, and to ensure proper handling of messages without content.
- Added checks to ensure that messages of the specified role are found in the transcript, with appropriate logging for missing messages.
- Refactored the logging mechanism to provide clearer insights into processing failures and successes.

src/sdk/prompts.ts

@@ -177,10 +177,10 @@ export function buildObservationPrompt(obs: Observation): string {
  * Build prompt to generate progress summary
  */
 export function buildSummaryPrompt(session: SDKSession): string {
-  const lastAssistantMessage = happy_path_error__with_fallback(
+  const lastAssistantMessage = session.last_assistant_message || happy_path_error__with_fallback(
     'Missing last_assistant_message in session for summary prompt',
     session,
-    session.last_assistant_message || ''
+    ''
   );
 
   return `PROGRESS SUMMARY CHECKPOINT