Ignore empty members of the Accept header

## Motivation and Context

An `Accept` header with an empty list member, such as `application/json,,text/event-stream` or one with
a leading comma, makes the Streamable HTTP transport answer a POST with 500 and report a `NoMethodError`
to the exception reporter, and raise out of a GET, although the required media types are listed
(issue #609): `parse_accept_header` splits each member on `;` and an empty member yields nothing to strip.
RFC 9110 section 5.6.1.2 has a recipient parse and ignore a reasonable number of empty list elements,
and the TypeScript and Python servers accept such a header, the one by substring checks and the other by
stripping each member.

`parse_accept_header` now skips empty members, so the POST and GET checks see the media types that are there.

Fixes #609.

## How Has This Been Tested?

Two new tests in `test/mcp/server/transports/streamable_http_transport_test.rb`, a POST with a doubled and
with a leading comma and a GET with a leading comma. Against the previous library the POST test fails with
a 500 and the GET test raises.

## Breaking Changes

None.

Author: koic

lib/mcp/server/transports/streamable_http_transport.rb

@@ -1883,10 +1883,10 @@ def validate_accept_header(request, required_types)
           nil
         end
 
+        # Empty list members (`application/json,,text/event-stream`), which RFC 9110 section 5.6.1.2 has
+        # a recipient parse and ignore, are skipped.
         def parse_accept_header(header)
-          header.split(",").map do |part|
-            part.split(";").first.strip.downcase
-          end
+          header.split(",").map { |part| part.split(";").first.to_s.strip.downcase }.reject(&:empty?)
         end
 
         def validate_content_type(request)

test/mcp/server/transports/streamable_http_transport_test.rb

@@ -1756,6 +1756,23 @@ def string
           assert_equal 200, response[0]
         end
 
+        test "POST request with empty members in the Accept header succeeds" do
+          ["application/json,,text/event-stream", ", application/json, text/event-stream"].each do |accept|
+            request = create_rack_request_without_accept(
+              "POST",
+              "/",
+              {
+                "CONTENT_TYPE" => "application/json",
+                "HTTP_ACCEPT" => accept,
+              },
+              { jsonrpc: "2.0", method: "initialize", id: "123", params: initialize_params }.to_json,
+            )
+
+            response = @transport.handle_request(request)
+            assert_equal 200, response[0], accept.inspect
+          end
+        end
+
         test "POST request with Accept: */* succeeds" do
           request = create_rack_request_without_accept(
             "POST",
@@ -1886,6 +1903,30 @@ def string
           assert_equal "text/event-stream", response[1]["content-type"]
         end
 
+        test "GET request with an empty member in the Accept header succeeds" do
+          init_request = create_rack_request(
+            "POST",
+            "/",
+            { "CONTENT_TYPE" => "application/json" },
+            { jsonrpc: "2.0", method: "initialize", id: "123", params: initialize_params }.to_json,
+          )
+          init_response = @transport.handle_request(init_request)
+          session_id = init_response[1]["mcp-session-id"]
+
+          request = create_rack_request(
+            "GET",
+            "/",
+            {
+              "HTTP_MCP_SESSION_ID" => session_id,
+              "HTTP_ACCEPT" => ",text/event-stream",
+            },
+          )
+
+          response = @transport.handle_request(request)
+          assert_equal 200, response[0]
+          assert_equal "text/event-stream", response[1]["content-type"]
+        end
+
         test "GET request with Accept: */* succeeds" do
           init_request = create_rack_request(
             "POST",