Related to #19465, WebMvc.fn SSE handling breaks when `spring.jackson.serialization.indent_output=true` is configured on Spring Boot side, likely because `MediaType.TEXT_EVENT_STREAM` is not visible from the Jackson converter [here](https://github.com/spring-projects/spring-framework/blob/69c8f8e9c7387d71ec1d08fb9d0b993f5b889866/spring-webmvc/src/main/java/org/springframework/web/servlet/function/SseServerResponse.java#L211), skipping [the SSE special handling code in `AbstractJackson2HttpMessageConverter`](https://github.com/spring-projects/spring-framework/blob/69c8f8e9c7387d71ec1d08fb9d0b993f5b889866/spring-web/src/main/java/org/springframework/http/converter/json/AbstractJackson2HttpMessageConverter.java#L478-L481).
Related to #19465, WebMvc.fn SSE handling breaks when
spring.jackson.serialization.indent_output=trueis configured on Spring Boot side, likely becauseMediaType.TEXT_EVENT_STREAMis not visible from the Jackson converter here, skipping the SSE special handling code inAbstractJackson2HttpMessageConverter.