feign-kotlin: add non-blocking WebClient AsyncClient and improve coroutine support - #3645

Summary

This PR combines coroutine support improvements for Feign and adds a Spring WebClient-backed non-blocking transport:

1. Fix suspend return types on the blocking builder (feign-kotlin: Decoder type returns "Object" when method is suspending #2019) — add KotlinDelegatingContract, which resolves Kotlin suspend return types with kotlin-reflect instead of exposing the JVM continuation signature (Object) to decoders. Regression tests cover this path.
2. Document non-blocking CoroutineFeign execution (How to use AsyncCient for non blocking requests? #2374) — explain that the default DefaultAsyncClient wraps a blocking client and show how to supply a non-blocking AsyncClient.
3. Support suspend methods returning Flow<T> (Support sse client #971) — decode the HTTP body as a buffered List<T> and expose it as a cold Flow. This is not incremental HTTP/SSE streaming.
4. Add feign-webflux — implement WebClientAsyncClient<C> as a Feign AsyncClient backed by Spring WebClient. It accepts a WebClient or WebClient.Builder, reusing configured connectors, filters, codecs and transport settings. With Reactor Netty, requests use its event-loop resources; a suspended caller does not park a thread while awaiting the response. The transport completes the future asynchronously, and response bodies are aggregated before Feign decoding.

Spring + CoroutineFeign usage

interface GitHubApi {
  @RequestLine("GET /users/{owner}/repos")
  suspend fun repositories(@Param("owner") owner: String): List<Repository>
}

@Configuration
class GitHubFeignConfiguration {
  @Bean
  fun gitHubApi(builder: WebClient.Builder): GitHubApi {
    @Suppress("UNCHECKED_CAST")
    return CoroutineFeign.builder<GitHubApi>()
      .client(WebClientAsyncClient<Any>(builder) as AsyncClient<GitHubApi>)
      .decoder(JacksonDecoder())
      .target(GitHubApi::class.java, "https://api.github.com")
  }
}

In Spring Boot WebFlux, the injected builder can carry the application's WebClient configuration. With the Reactor Netty connector, the application and client can share Reactor Netty's default event-loop resources. This adds manual Spring bean integration, not @FeignClient auto-configuration for spring-cloud-openfeign.

Behavior and compatibility notes

• feign-webflux requires Java 17+ (Spring Framework 7 baseline). Adding this optional module does not change Feign Core's Java baseline.
• A new integration test delays the HTTP response while using a single-thread caller dispatcher, then verifies another coroutine task runs on that dispatcher before the response arrives. It also verifies a filter configured on the supplied WebClient.Builder reaches the request.
• The test demonstrates that the caller dispatcher is not blocked while suspended; it does not mean no thread is active at all. Reactor Netty event-loop threads still perform I/O and completion work.
• Response bodies are buffered before decoding; neither this adapter nor Flow<T> provides true streaming.
• Per-request Feign read timeout is applied as an overall reactive timeout that cancels the request. Feign per-request connect timeout and redirect options are not translated; configure these on the WebClient connector.
• Decoding runs on the future completion thread, typically the Reactor Netty event loop. Keep decoders and completion callbacks short and non-blocking.
• Default Reactor Netty resources are shared. The application is responsible for lifecycle/disposal of custom Netty resources or connectors it supplies.
• Non-success HTTP statuses remain Feign Response values for ErrorDecoder handling.

Testing

• Full required reactor: ./mvnw -pl webflux -am test -Dtoolchains.skip=true -Dtoolchain.skip=true -Dmaven.javadoc.skip=true -Dlicense.skip=true — 898 tests, 0 failures/errors, 4 skipped across core, Jackson, Java 11, Kotlin and WebFlux.
• WebFlux module: 9 tests passed, including the delayed-response coroutine suspension test.
• ./mvnw -pl webflux license:check -Dtoolchains.skip=true -Dtoolchain.skip=true -Dmaven.javadoc.skip=true — passed.

Fixes #2019
Fixes #2374
Closes #971 (buffered Flow semantics; true streaming left for discussion)