fix: Ensure UI feedback message for batch processing completion

[fg-nava]

fix: resolve missing batch processing UI feedback message

Ticket

https://navalabs.atlassian.net/browse/DST-709

Changes

• Updated message formatting in chainlit.py to ensure completion status is visible in UI
• Added proper metadata tracking for processing status
• Made completion message more descriptive by including result filename

Context for reviewers

The batch processing feature was not showing completion messages in the UI, even though they were being logged successfully. Investigation showed that messages with author "backend" were not consistently visible in the Chainlit UI.

Key changes:

• Added metadata to include processing status and file information
• Made completion message more informative by including the output filename
• Ensured consistent message visibility throughout the processing flow

This PR is part of improving the batch processing feature's user experience, following the fix for CPU usage issues in DST-688.

Testing

Tested locally with the following scenarios:

1. Uploaded a CSV file with valid questions

   • Confirmed "File processed, results attached" message appears with download link
   • Verified file downloads successfully

2. Uploaded a CSV without a "question" column

   • Confirmed error message appears with proper formatting

3. Verified all status messages appear in sequence:

   • "Please upload a CSV file..."
   • "Received file, processing..."
   • "File processed, results attached"

4. Screenshot:

   

[github-actions]

☂️ Python Coverage

current status: ✅

Overall Coverage

Lines	Covered	Coverage	Threshold	Status
2845	2543	89%	80%	🟢

New Files

No new covered files...

Modified Files

File	Coverage	Status
app/src/batch_process.py	100%	🟢
TOTAL	100%	🟢

updated for commit: b3c3886 by action🐍