Support application/json responses in Streamable HTTP transport (opt-in JSON response mode)

Expected Behavior

An opt-in configuration to return application/json for JSON-RPC requests within the Streamable HTTP transport, even with active sessions. Example:

HttpServletStreamableServerTransportProvider.builder()
    .jsonMapper(mapper)
    .mcpEndpoint("/mcp")
    .jsonResponse(true)    // opt-in, default false
    .build();

When enabled, doPost() for JSONRPCRequest would buffer the response from session.responseStream() and return it as application/json instead of opening an SSE stream. Server-initiated notifications would go through the standalone SSE stream (GET on the same /mcp endpoint, as defined in the Streamable HTTP spec).

All other official MCP SDKs already support this as an opt-in option:

Current Behavior

In HttpServletStreamableServerTransportProvider.doPost(), session-bound JSONRPCRequest messages always get text/event-stream:

else if (message instanceof McpSchema.JSONRPCRequest jsonrpcRequest) {
    // For streaming responses, we need to return SSE
    response.setContentType(TEXT_EVENT_STREAM);   // ← always SSE
    // ...
}

There is no way to get application/json for session-bound requests. The initialize request (line 461) and HttpServletStatelessServerTransport (line 173) already return application/json - so the pattern exists in the codebase.

Context

The MCP Streamable HTTP spec (2025-06-18, §2.1.5) states:

"If the input is a JSON-RPC request, the server MUST either return Content-Type: text/event-stream [...] or Content-Type: application/json, to return one JSON object."

This means session management does not require SSE for every response. There are scenarios where a server benefits from sessions (capability negotiation, logging, server-initiated notifications via GET) but its tool handlers produce simple single-message responses that don't need SSE framing.

The existing STATELESS mode (HttpServletStatelessServerTransport) does return application/json, but it removes session management entirely - no server-to-client notifications, no sampling, no capability tracking. This feature request is about supporting application/json responses within the Streamable transport, with sessions, as the spec allows and as the other SDKs already implement.

In local benchmarks comparing MCP server implementations across different SDKs, we observed that switching from SSE-only to JSON responses for simple tool calls (single request-response, no intermediate notifications) resulted in significant performance improvements, primarily due to eliminating chunked transfer encoding, AsyncContext lifecycle overhead, and SSE framing/parsing on both server and client sides.

Would it make sense to add this? Happy to contribute a PR if the maintainers agree.

从 HttpServletStreamableServerTransportProvider.doPost() 开始，将其会话绑定的 JSON-RPC 路径与 initialize 路径和 HttpServletStatelessServerTransport 中对 application/json 的处理进行比较。完成的标准是：可选择启用的 jsonResponse 设置默认为 false，启用后为会话绑定的请求返回缓冲的 JSON，并保留用于服务器通知的独立 SSE 路径。