feat: prevent UI from showing raw JSON when AI response is empty

[Pavilion4ik]

What

• Fixed an issue where raw JSON responses were shown only in the non-streaming flow
• Improved backend LLM response handling to properly fallback when response values are empty
• Replaced dict.get(key, default) with explicit value-or-default logic
• Added a shared frontend constant for no-response scenarios
• Updated frontend logic to display a user-friendly fallback message

Why

• The issue affected non-streaming responses only, since streaming mode yields plain text chunks and does not operate on JSON payloads
• In the non-streaming flow, dict.get(key, default) returned empty values when keys existed but contained no data
• This caused empty or unreadable LLM responses to reach the frontend and be rendered as raw JSON
• The change ensures consistent backend behavior and improves UX by always showing a clear fallback message

[openedx-webhooks]

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.58%. Comparing base (b200e96) to head (2b43355).

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #95   +/-   ##
=======================================
  Coverage   90.58%   90.58%           
=======================================
  Files          47       47           
  Lines        4310     4310           
  Branches      271      271           
=======================================
  Hits         3904     3904           
  Misses        317      317           
  Partials       89       89           

Flag	Coverage Δ
unittests	90.58% <ø> (ø)

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[felipemontoya]

@Pavilion4ik the PR makes sense. Where you able to consistently reproduce the error? I know it happened for me sometimes, but I could not tell if it was related to streaming+functions at the same time

[felipemontoya]

For consistency I think we should leave it the same as the backend: "No response available"