Add defaultAuthenticationEntryPointFor to Reactive ExceptionHandlingSpec

[ZaMan0806]

This PR adds ServerHttpSecurity.ExceptionHandlingSpec#defaultAuthenticationEntryPointFor(ServerAuthenticationEntryPoint, ServerWebExchangeMatcher), aligning it with ExceptionHandlingConfigurer#defaultAuthenticationEntryPointFor on the servlet side. It also adds the equivalent to ServerExceptionHandlingDsl for the Kotlin DSL.

Closes gh-9663

Ordering

Built-in features register their defaults in ServerHttpSecurity#defaultEntryPoints at build time, and formLogin inserts its entry at index 0. If user-registered entries were simply appended to that list, formLogin's HTML matcher would take precedence over them. To keep user-registered entries in charge, which matches the servlet behavior where they are registered before the built-in ones, the spec keeps its own list, and that list is consulted first. When nothing matches, the fallback stays the same as today (the last default).

requestWhenDefaultAuthenticationEntryPointForAndFormLoginThenPreferredEntryPointTakesPrecedence covers this case. I confirmed that it fails if the entries are appended directly to defaultEntryPoints.

An explicitly configured authenticationEntryPoint(...) still overrides all defaults, and nothing changes when defaultAuthenticationEntryPointFor is not used.

I'm happy to adjust the ordering if a different behavior is preferred.

Tests

• ExceptionHandlingSpecTests: matching and non-matching requests, precedence over formLogin, explicit authenticationEntryPoint override, and null argument checks
• ServerExceptionHandlingDslTests: Kotlin DSL support