Fix: Add null check and error handling for JSON parsing in /decode endpoint

[prjanitor]

Summary

This PR fixes a bug in the /decode endpoint where json.loads() was called directly on the input parameter without checking if it was None or empty. This caused the server to crash with a TypeError or JSONDecodeError when the 'input' query parameter was missing or empty.

Changes

• Added null/empty check for the input parameter before JSON parsing
• Wrapped json.loads() in a try-except block to catch JSONDecodeError
• Returns appropriate 400 error responses with descriptive error messages

Testing

The endpoint now properly handles:

• Missing 'input' parameter → Returns 400 with error message
• Empty 'input' parameter → Returns 400 with error message
• Invalid JSON → Returns 400 with error message
• Valid JSON → Processes normally

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