Client request handlers that complete empty send no JSON-RPC response

Bug description

When a server sends a request to a Java SDK client, an asynchronous sampling or elicitation handler can return Mono.empty(). McpClientSession then sends no JSON-RPC response, so the server waits until its request timeout.

JSON-RPC 2.0 requires a response for every request with an ID. An empty handler completion should produce an error response with the original request ID.

This is the client-side counterpart to https://github.com/modelcontextprotocol/java-sdk/issues/1081. The open fix at https://github.com/modelcontextprotocol/java-sdk/pull/1099 changes only server-side dispatch paths.

Environment

• Java SDK v2.0.1, commit c7e1cfe90edcd9cbe030924310a194dc5492eab2
• Java SDK main, commit 39c225e46c51311eaf82192f15b950e421580fb5
• Temurin 25.0.3, compiling with the repository's Java 17 target

Steps to reproduce

Register a client request handler that returns Mono.empty(), then deliver a JSON-RPC request with an ID:

var transport = new MockMcpClientTransport();
var session = new McpClientSession(TIMEOUT, transport,
		Map.of("test.method", params -> Mono.empty()), Map.of(), Function.identity());

transport.simulateIncomingMessage(new McpSchema.JSONRPCRequest("test.method", "test-id"));

assertThat(transport.getLastSentMessage()).isInstanceOf(McpSchema.JSONRPCResponse.class);

Both v2.0.1 and main fail because getLastSentMessage() is null:

Tests run: 1, Failures: 1, Errors: 0, Skipped: 0
Expecting actual not to be null

Expected behavior

The client sends one JSON-RPC error response with ID test-id and code -32603.

Minimal Complete Reproducible example

The regression test is in McpClientSessionTests.testEmptyRequestHandlerSendsErrorResponse.

Start with McpClientSession and the regression test McpClientSessionTests.testEmptyRequestHandlerSendsErrorResponse, using MockMcpClientTransport to reproduce the empty-handler case. Trace incoming JSON-RPC request dispatch and verify that Mono.empty() produces one JSON-RPC error response with request ID test-id and code -32603.