fix: only log 4xx errors when debug is enabled in DebuggableExceptionMapper

[VinayakGote (Vinu2111)]

Problem

When debug=false, 4xx errors (such as RestNotFoundException) were still
being logged via log.debug(...) unconditionally in DebuggableExceptionMapper.
The comment in the code said "Only log other errors, including user-errors 4xx,
if debugging enabled" — but the condition was never actually checked for logging.

Fixes the behaviour described in #221.

Change

In createResponse(), the else branch that logs 4xx errors has been changed
to an else if that checks restConfig.getBoolean(RestConfig.DEBUG_CONFIG)
before logging. 5xx errors are unaffected — they always log at ERROR level.

Before:

} else {
  log.debug("Request Failed with exception ", exc);
}

After:

} else if (restConfig != null && restConfig.getBoolean(RestConfig.DEBUG_CONFIG)) {
  log.debug("Request Failed with exception ", exc);
}

Tests

Added DebuggableExceptionMapperTest covering three cases:

• 4xx with debug=false → no stack trace in response message
• 4xx with debug=true → stack trace included in response message
• 5xx with debug=false → always logs, no stack trace in message