Send value as a float to ai_response_time event

MR fixes https://new-sentry.gitlab.net/organizations/gitlab/issues/1395828/?alert_rule_id=38&alert_type=issue&environment=gprd&notification_uuid=8b77c6c6-a19b-4dfb-9571-2fc192615a3c&project=3&referrer=slack

This change fixes a potential issue with tracking AI response time. Instead of directly using the duration value, the code now ensures it's converted to a floating-point number using parseFloat(), with a fallback to 0 if the conversion fails. This prevents potential errors that could occur if duration was in an unexpected format or couldn't be properly interpreted as a number.