Forward dismiss404 from CoroutineFeign builder to AsyncFeign

[seonwoo-jung]

CoroutineFeign.CoroutineBuilder inherits dismiss404() from BaseBuilder, but internalBuild() constructed a fresh AsyncFeign.builder() chain that never forwarded the flag. As a result, calling .dismiss404() on a coroutine builder silently had no effect — a 404 response still produced a FeignException instead of being routed through the decoder.

Fix

Split the existing builder chain so the configured dismiss404 value is applied to the underlying AsyncBuilder before build():

AsyncFeign.AsyncBuilder<Object> asyncBuilder =
    AsyncFeign.builder()
        .logLevel(logLevel)
        ...
        .methodInfoResolver(methodInfoResolver);
if (dismiss404) {
  asyncBuilder.dismiss404();
}
AsyncFeign<C> asyncFeign = (AsyncFeign<C>) asyncBuilder.build();

The conditional mirrors BaseBuilder.dismiss404() semantics (a one-way switch); preserving the default false behavior when the user doesn't opt in.

Test

Adds CoroutineFeignTest.\sut should dismiss 404 responses when dismiss404 is configured on CoroutineBuilder`, modeled on AsyncFeignTest.decodingDoesNotSwallow404ErrorsInDismiss404Mode. The test enqueues a 404, installs a recording Decoder`, and asserts that:

1. The call does not throw a FeignException.
2. The decoder is invoked with response.status() == 404, proving the dismiss-404 path engaged.

Without the fix, the call would throw on the 404 before the decoder is ever called.

Fixes #2026