fix: crash when parsing invalid json config

[greghuels]

Eppo Internal:

🎟️ Fixes FFESUPPORT-458

Summary

Fixes #51 - crash in parseConfigResponse when receiving invalid or non-object JSON input.

• Added validation to check that input JSON is an object before calling contains() method
• Added comprehensive test coverage for malformed input edge cases

Root Cause

The parseConfigResponse(const nlohmann::json& j) function called j.contains("flags") without first validating that j is a JSON object. In nlohmann::json, calling contains() on a non-object type (null, array, number, string, boolean) throws a type_error.306 exception, causing the EXCEPTION_ACCESS_VIOLATION_READ crash reported in the issue.

This could occur when the backend returns empty, corrupt, or unexpected responses.

Test plan

• Added 11 new test cases covering edge cases:
  • Empty string input
  • Null JSON input (both as json object and string "null")
  • Array JSON input (both as json object and string "[]")
  • Number JSON input
  • String JSON input
  • Boolean JSON input
  • Malformed JSON string
  • Truncated JSON string
• All 125 existing tests continue to pass

[leoromanovsky]

consider another log line that prints out the received configuration to give them a totally self service way to iterate; alternatively does this error give them a strong enough signal that they could catch and print it themselves?

[greghuels]

The current error message includes the JSON type name (e.g., "Expected a JSON object, got null" or "got array"), which should give a good signal about what happened. Since parseConfigResponse returns a ParseResult with the error in result.errors, consuming apps can catch and log the full input themselves. So we should be good 👍