Validate Content-Type on POST requests

## Motivation and Context

The MCP Streamable HTTP specification requires POST request bodies to
be JSON-RPC messages with `Content-Type: application/json`. The Ruby
SDK did not validate Content-Type, accepting requests with any or no
Content-Type. The Python SDK validates this and returns 415
Unsupported Media Type for non-JSON Content-Types.

## How Has This Been Tested?

Added tests for Content-Type validation:

- POST without Content-Type returns 415
- POST with `Content-Type: text/plain` returns 415
- POST with `Content-Type: application/json; charset=utf-8` succeeds

## Breaking Changes

POST requests without `Content-Type: application/json` now return
415 Unsupported Media Type. This is a spec compliance fix.

lib/mcp/server/transports/streamable_http_transport.rb

@@ -267,6 +267,9 @@ module MCP
           accept_error = validate_accept_header(request, REQUIRED_POST_ACCEPT_TYPES)
           return accept_error if accept_error
 
+          content_type_error = validate_content_type(request)
+          return content_type_error if content_type_error
+
           body_string = request.body.read
           session_id = extract_session_id(request)
 
@@ -399,6 +402,18 @@ module MCP
           end
         end
 
+        def validate_content_type(request)
+          content_type = request.env["CONTENT_TYPE"]
+          media_type = content_type&.split(";")&.first&.strip&.downcase
+          return if media_type == "application/json"
+
+          [
+            415,
+            { "Content-Type" => "application/json" },
+            [{ error: "Unsupported Media Type: Content-Type must be application/json" }.to_json],
+          ]
+        end
+
         def not_acceptable_response(required_types)
           [
             406,