Feat: Preserve JSON view when jq returns null or fails

[ynqa]

When a jq query returns only null values or results in an error, the response pane should continue showing meaningful (something) JSON view instead of becoming empty or losing context.

This should:

• Keep the response pane stable to grasp how to fix it during invalid/unsuccessful queries
• Show warning/error information in the guide pane as it does today
• Maintain existing behavior for successful queries

Methodology

• Return the originally provided JSON
• Return the result of the most recent successful query
• Return the result of the longest valid prefix of the current query