Fix tool call failed when tool name is Chinese.

[pcalpha]

Bug description
When I call the tool which name is Chinese, The mcp client will throws Exception below.

Exception in thread "main" io.modelcontextprotocol.spec.McpError: Tool not found: 天���
	at io.modelcontextprotocol.spec.McpClientSession.lambda$sendRequest$11(McpClientSession.java:248)
	Suppressed: The stacktrace has been enhanced by Reactor, refer to additional information below:

The tool name is messy code.

Environment
Spring AI 1.0.0 , mcp 0.10.0 , without vector store

Java SDK
Openjdk17

Steps to reproduce
Just add a tool with a Chinese name(such as 天气预报), then call the tool.

Recommendation
sendMessage in HttpClientSseClientTransport.java

String jsonText = this.objectMapper.writeValueAsString(message);
			URI requestUri = Utils.resolveUri(baseUri, endpoint);
			HttpRequest request = this.requestBuilder.uri(requestUri)
				.setHeader("Content-Type", "application/json;charset=UTF-8")
				.POST(HttpRequest.BodyPublishers.ofString(jsonText))
				.build();

So just set the default request encoding of HttpRequest in the HttpClientSseClientTransport to UTF-8.