Add timeout and fallback logging for closeGracefully() to prevent hanging shutdown

先阅读 McpAsyncClient.closeGracefully()，然后检查 initializer.closeGracefully() 和 transport.closeGracefully()，以了解它们的完成行为。确认超时处理、警告和作为 fallback 的强制关闭、错误处理以及最终完成能够使关闭过程有界，同时不改变预期的 graceful 路径。

Problem

When using McpAsyncClient.closeGracefully(), the client executes:

return this.initializer.closeGracefully()
        .then(transport.closeGracefully());

Both initializer.closeGracefully() and transport.closeGracefully() return Mono<Void>.
However, if either of them hangs—for example:

• The underlying transport (HTTP/SSE/WebSocket) never completes
• The server doesn’t respond to shutdown
• A Reactor pipeline remains open (no onComplete)

then the returned Mono never completes, causing the application to hang indefinitely during shutdown.

This results in JVMs or containers that never terminate, blocking CI/CD or production deployments.

Goal

Add a timeout and fallback mechanism to ensure that the client always terminates safely, even when the transport or initializer fails to complete.

Proposed Change

1. Wrap shutdown calls with timeout and fallback

Use Reactor’s timeout(Duration, fallbackMono) operator to guarantee a bounded shutdown duration.

public Mono<Void> closeGracefully() {
    return Mono.defer(() -> {
        long start = logger.isDebugEnabled() ? System.nanoTime() : 0L;
        Duration timeout = Duration.ofSeconds(
                Integer.getInteger("mcp.shutdown.timeout.seconds", 10));

        Mono<Void> graceful = this.initializer.closeGracefully()
            .then(transport.closeGracefully());

        Mono<Void> fallback = Mono.fromRunnable(() -> {
                logger.warn("closeGracefully() timed out after {} seconds; proceeding with best-effort shutdown.", timeout.getSeconds());
                try {
                    this.transport.close(); // force-close if needed
                } catch (Throwable t) {
                    logger.warn("Fallback forced close encountered error: {}", t.toString());
                }
            })
            .then();

        return graceful
            .timeout(timeout, fallback)
            .doOnError(e -> logger.warn("closeGracefully() failed: {}", e.toString()))
            .onErrorResume(e -> Mono.empty()) // ensure app doesn't hang
            .doFinally(sig -> {
                if (logger.isDebugEnabled()) {
                    long durationMs = (System.nanoTime() - start) / 1_000_000;
                    logger.debug("closeGracefully() finished with signal={}, took {} ms", sig, durationMs);
                }
            });
    });
}

Summary

Introduce a timeout and fallback mechanism for closeGracefully() to guarantee reliable termination, preventing hanging shutdowns when the transport or lifecycle initializer fails to complete.