fix: keep connection open after responseStream to enable keep-alive ping

[JGoP-L]

Problem

When a Streamable HTTP client (e.g. Cursor) sends a JSON-RPC request such as
tools/list without establishing a separate GET SSE listening stream,
responseStream() calls transport.closeGracefully() immediately after sending
the response. This closes the SSE connection, preventing KeepAliveScheduler
from sending ping messages through it.

As a result, clients that rely on keep-alive pings to detect connection health
(e.g. Cursor) mark the MCP server as disconnected (red state) after the idle
timeout period.

Fixes #681

Root Cause

In McpStreamableServerSession.responseStream(), after sending the response:

• The normal handler path unconditionally called .then(transport.closeGracefully()),
  closing the SSE connection immediately.
• The method-not-found path returned after sendMessage() without promoting
  the transport to a listening stream.

In both cases, listeningStreamRef remains pointing to MissingMcpTransportSession,
causing KeepAliveScheduler ping attempts to fail with "Stream unavailable".

[Copilot]

Pull request overview

This PR fixes an issue where Streamable HTTP clients that don't establish a separate GET SSE listening stream (e.g., Cursor IDE) would be marked as disconnected after idle timeout. The root cause was that responseStream() would close the connection immediately after sending a response, preventing KeepAliveScheduler from sending ping messages.

Changes:

• Modified responseStream() to conditionally promote the response stream to a listening stream when no listening stream exists
• Applied the fix to both the error handler path (method-not-found) and the success handler path

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[Copilot]

The logic for promoting a response stream to a listening stream is duplicated in both the error handler path (lines 175-184) and the success handler path (lines 202-211). Consider extracting this into a private helper method to reduce duplication and improve maintainability. For example:

private Mono<Void> promoteToListeningStreamOrClose(
    McpStreamableServerSessionStream stream, 
    McpStreamableServerTransport transport) {
    return Mono.defer(() -> {
        McpLoggableSession currentListeningStream = this.listeningStreamRef.get();
        if (currentListeningStream == this.missingMcpTransportSession) {
            if (this.listeningStreamRef.compareAndSet(this.missingMcpTransportSession, stream)) {
                logger.debug("Converted response stream to listening stream for session {}", this.id);
                return Mono.empty();
            }
        }
        return transport.closeGracefully();
    });
}

This would make the code easier to maintain and ensure consistent behavior across both paths.