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.

plugin/scripts/worker-service.cjs

@@ -781,7 +781,7 @@ MEMORY PROCESSING START
 Write progress notes of what was done, what was learned, and what's next. This is a checkpoint to capture progress so far. The session is ongoing - you may receive more requests and tool executions after this summary. Write "next_steps" as the current trajectory of work (what's actively being worked on or coming up next), not as post-session future work. Always write at least a minimal summary explaining current progress, even if work is still in early stages, so that users see a summary output tied to each request.
 
 Claude's Full Response to User:
-${vr("Missing last_assistant_message in session for summary prompt",a,a.last_assistant_message||"")}
+${a.last_assistant_message||vr("Missing last_assistant_message in session for summary prompt",a,"")}
 
 Respond in this XML format:
 <summary>