feat: Set $feature_flag_error on $feature_flag_called

[dustinbyrne]

This change adds observability into remote 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 (curl errno 28)
connection_error	Network connectivity issues (DNS failure, connection refused, etc.)
api_error_{status}	API returned HTTP error with status code (e.g., api_error_500)
errors_while_computing_flags	Server returned errorsWhileComputingFlags: true
quota_limited	Feature flags quota exceeded (quotaLimited contains feature_flags)
flag_missing	Requested flag not in API response
unknown_error	Unexpected error that doesn't match known patterns

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

[dustinbyrne]

Something changed either on the runner or the shivammathur/setup-php@v2 action that's now failing PHP 8.4. As far as I can tell, it's unrelated to this change.