Fix crash when HttpResponseDecoder receives non-JSON response

[hanselip]

Summary

This PR fixes issue #1338 where the application crashes when HttpResponseDecoder::processBody receives a non-JSON HTTP response (e.g., HTML from captive portals or network sign-in pages).

Changes

• Added validation before JSON parsing in processBody method
• Uses nlohmann::json::accept() to validate response body before attempting to parse
• Logs an error and returns early if the response is not valid JSON, preventing crashes

Problem

When a network request returns a non-JSON response (such as HTML from a captive portal), the library attempts to parse it as JSON, causing a crash in the nlohmann::json::parse() function.

Solution

The fix adds a check using nlohmann::json::accept() which safely validates whether the response body is valid JSON before attempting to parse it. If validation fails, the function logs an error and returns early.

Testing

• The fix uses the built-in nlohmann::json::accept() method which is designed for safe validation
• The try-catch block still provides additional protection for any unexpected parsing errors
• CI tests will validate that existing functionality remains intact

Fixes #1338