transportContext is the default empty context McpTransportContext.EMPTY because WebMvcStreamableServerTransportProvider.Builder.contextExtractor is not set

when i use ToolContext in tool method i always get a null value from transportContext , and i found that the McpTransportContext.EMPTY has set by default in WebMvcStreamableServerTransportProvider Builder class

issue description
WebMvcStreamableServerTransportProvider initialized in McpServerStreamableHttpWebMvcAutoConfiguration like this :

   public WebMvcStreamableServerTransportProvider webMvcStreamableServerTransportProvider(
			ObjectProvider<ObjectMapper> objectMapperProvider, McpServerStreamableHttpProperties serverProperties) {

		ObjectMapper objectMapper = objectMapperProvider.getIfAvailable(ObjectMapper::new);

		return WebMvcStreamableServerTransportProvider.builder()
			.jsonMapper(new JacksonMcpJsonMapper(objectMapper))
			.mcpEndpoint(serverProperties.getMcpEndpoint())
			.keepAliveInterval(serverProperties.getKeepAliveInterval())
			.disallowDelete(serverProperties.isDisallowDelete())
			.build();
	}

but whitout setting contextExtractor , this leads to a default empty McpTransportContext set in WebMvcStreamableServerTransportProvider

Environment
spring-ai-bom version is 1.1.0-M1
modelcontextprotocol java-sdk version is 0.12.1

    public static class Builder {
                // default empty McpTransportContext 
		private McpTransportContextExtractor<ServerRequest> contextExtractor = (serverRequest) -> McpTransportContext.EMPTY;
                .......
     }

Steps to reproduce

1. set a header named 'xxxx' with value '123456' for streamable request
2. get header value from ToolContext.transportContext

Expected behavior
i expect to get the value 123456 of header 'xxxx' from transportContext but a null actually

Can I Add the ContextExtractor Setting in the WebMvcStreamableServerTransportProvider initialize just like this :

@Bean
	@ConditionalOnMissingBean
	public WebMvcStreamableServerTransportProvider webMvcStreamableServerTransportProvider(
			ObjectProvider<ObjectMapper> objectMapperProvider, McpServerStreamableHttpProperties serverProperties) {

		ObjectMapper objectMapper = objectMapperProvider.getIfAvailable(ObjectMapper::new);

                // begin
                McpTransportContextExtractor<ServerRequest> contextExtractor = (serverRequest) -> {
                     Map<String, Object> headersMap = new HashMap<>();
                     HttpServletRequest httpServletRequest = serverRequest.servletRequest();
                     Enumeration<String> headerNames = httpServletRequest.getHeaderNames();
                     while (headerNames.hasMoreElements()) {
                         String headerName = headerNames.nextElement();
                         String headerVal = httpServletRequest.getHeader(headerName);
                         headersMap.put(headerName, headerVal);
                     }
                    return McpTransportContext.create(headersMap);
                };
                // end

		return WebMvcStreamableServerTransportProvider.builder()
			.jsonMapper(new JacksonMcpJsonMapper(objectMapper))
			.mcpEndpoint(serverProperties.getMcpEndpoint())
			.keepAliveInterval(serverProperties.getKeepAliveInterval())
			.disallowDelete(serverProperties.isDisallowDelete())
                        .contextExtractor(contextExtractor ) // set customize contextExtractor 
			.build();
	}