HTTP client fails connect when the server acknowledges notifications/initialized with an empty 200 #598

Observed behavior

With mcp 1.7.0 (and 0.15.0), MCP::Client#connect with the legacy initialize handshake over Streamable HTTP fails against servers that acknowledge notifications/initialized with 200 OK, an empty body and no Content-Type, instead of 202 Accepted:

MCP::Client::RequestHandlerError: Unsupported Content-Type: nil. Expected application/json or text/event-stream.
error_type: :unsupported_media_type

The initialize request succeeds. The error comes from the following notifications/initialized POST: send_request passes the response to resolve_response_body, which returns early only for 202 and raises for any other response without a JSON or SSE content type.

For example, Freshservice's hosted MCP server (Freshworks, https://<account>.freshservice.com/mcp) responds this way:

POST /mcp  {"jsonrpc":"2.0","method":"notifications/initialized"}

HTTP/1.1 200 OK
Content-Length: 0

I ran each SDK's client against the same Freshservice server, with the same OAuth token, on 2026-10-13:

Expected behavior

The spec says the server MUST return 202 Accepted with no body when it accepts a notification (Transports, Sending Messages to the Server), so the server is out of spec here. Still, a notification awaits no response, and both reference SDKs accept any 2xx for it:

• TypeScript SDK 1.32.1: after the 202 check, the content type is checked only when the POST carried a request. For a notification, any other 2xx reaches the // No requests in message but got 200 OK - still need to release connection branch, which cancels the body and returns.
• Python SDK 2.3.0: after the 202 check, the content-type handling is gated on isinstance(message, JSONRPCRequest), with the comment "The server MUST NOT send a response to notifications." A notification answered with 2xx falls through without reading the body.

In both, a request answered the same way is still an error: TypeScript throws StreamableHTTPError and Python returns a JSON-RPC error, each with "Unexpected content type".

I'd expect connect to succeed when a notification is acknowledged with a 2xx and no body. Requests should keep raising unsupported_media_type for the same response.

Reproduction

require "mcp"
require "webmock"
include WebMock::API
WebMock.enable!

url = "https://example.com/mcp"
stub_request(:post, url)
  .with { |req| JSON.parse(req.body)["method"] == "initialize" }
  .to_return(
    status: 200,
    headers: { "Content-Type" => "application/json" },
    body: { jsonrpc: "2.0", id: "1", result: { protocolVersion: "2025-11-25", capabilities: {}, serverInfo: { name: "s", version: "1" } } }.to_json,
  )
stub_request(:post, url)
  .with { |req| JSON.parse(req.body)["method"] == "notifications/initialized" }
  .to_return(status: 200, body: "")

MCP::Client.new(transport: MCP::Client::HTTP.new(url: url)).connect(mode: :legacy)
# => MCP::Client::RequestHandlerError: Unsupported Content-Type: nil. ...

Fix: #599. It returns nil instead of raising in that branch when the message is a notification and the status is 2xx, matching the behaviour above.