Write ResponseStatusException headers in WebFlux error responses

[U-S-jun]

AbstractErrorWebExceptionHandler renders a ResponseStatusException before
Spring Framework's ResponseStatusExceptionHandler can apply its response
headers. As a result, exception-contributed headers such as Allow on a 405
response are lost.

This change applies headers contributed by a direct ResponseStatusException
before writing the generated ServerResponse, matching Spring Framework's
existing behavior. It deliberately leaves the broader error-handling
infrastructure unchanged.

Closes gh-18831

Tests:

• DefaultErrorWebExceptionHandlerIntegrationTests.responseStatusExceptionHeaders
• :module:spring-boot-webflux:test
• checkFormatMain, checkFormatTest, checkstyleMain, and checkstyleTest
• git diff --check

[U-S-jun]

Rebased onto the current main and re-ran DefaultErrorWebExceptionHandlerIntegrationTests.responseStatusExceptionHeaders with JDK 25.

The change remains intentionally limited to propagating headers from a direct ResponseStatusException, matching Spring Framework behavior while leaving the broader error-handling infrastructure unchanged. I am happy to adjust the scope if the team would prefer a broader change.