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

[ThomsonTan]

The wrapper try/catch seems no longer needed with this validation?

[hanselip]

Thanks! With the validation approach using parse() with allow_exceptions=false, the try/catch wrapper is no longer needed. I've removed it in the latest commit.

The function now handles invalid JSON gracefully through return value checking rather than exception handling, making the code cleaner and more efficient.

Changes pushed in commit ab8790c.