feat: Set $feature_flag_error on $feature_flag_called

[dustinbyrne]

This change adds observability into feature flag evaluation failures by including a $feature_flag_error property on $feature_flag_called events. The error types are:

Error Type	Description
timeout	Request timed out (context deadline, cancellation, net.Error timeout)
connection_error	Network connectivity issues (DNS errors, connection refused)
errors_while_computing_flags	Server returned errorsWhileComputingFlags: true
quota_limited	Feature flags quota exceeded
flag_missing	Requested flag not in API response
api_error_*	HTTP error status codes (e.g., api_error_500, api_error_503)
unknown_error	Unexpected error that doesn't match known patterns

Multiple errors can occur simultaneously and are joined with commas (e.g., timeout,flag_missing).

[haacked]

Nice work!

The unit tests for classifyError() and GetErrorString() are excellent, but I don't see integration tests verifying that $feature_flag_error actually gets set on captured events. Maybe add a test following the pattern in feature_flags_flags_test.go?