Return 404 for invalid session ID in handle_delete

## Motivation and Context

The MCP specification requires that when a server receives a request containing
a session ID that is no longer valid, it MUST respond with HTTP 404 Not Found.
`handle_delete` was unconditionally calling `cleanup_session` and returning 200
even for nonexistent session IDs.

Ref: https://modelcontextprotocol.io/specification/2025-11-25/basic/transports#session-management

> The server MAY terminate the session at any time, after which it MUST respond
> to requests containing that session ID with HTTP 404 Not Found.

## How Has This Been Tested?

Added tests for DELETE with invalid session ID, and lifecycle tests for
delete-then-POST and delete-then-DELETE scenarios.

## Breaking Change

DELETE requests with an invalid session ID now return HTTP 404 "Session not found"
instead of HTTP 200.

However, this change is considered a bug fix because it brings the behavior into
compliance with the MCP specification.

test/mcp/server/transports/streamable_http_transport_test.rb

@@ -332,6 +332,84 @@ module MCP
           assert body["success"]
         end
 
+        test "handles DELETE request with invalid session ID" do
+          request = create_rack_request(
+            "DELETE",
+            "/",
+            { "HTTP_MCP_SESSION_ID" => "invalid_id" },
+          )
+
+          response = @transport.handle_request(request)
+          assert_equal 404, response[0]
+          assert_equal({ "Content-Type" => "application/json" }, response[1])
+
+          body = JSON.parse(response[2][0])
+          assert_equal "Session not found", body["error"]
+        end
+
+        test "POST returns 404 after session is deleted" do
+          init_request = create_rack_request(
+            "POST",
+            "/",
+            { "CONTENT_TYPE" => "application/json" },
+            { jsonrpc: "2.0", method: "initialize", id: "init" }.to_json,
+          )
+          init_response = @transport.handle_request(init_request)
+          session_id = init_response[1]["Mcp-Session-Id"]
+
+          delete_request = create_rack_request(
+            "DELETE",
+            "/",
+            { "HTTP_MCP_SESSION_ID" => session_id },
+          )
+          @transport.handle_request(delete_request)
+
+          post_request = create_rack_request(
+            "POST",
+            "/",
+            {
+              "CONTENT_TYPE" => "application/json",
+              "HTTP_MCP_SESSION_ID" => session_id,
+            },
+            { jsonrpc: "2.0", method: "ping", id: "456" }.to_json,
+          )
+          response = @transport.handle_request(post_request)
+          assert_equal 404, response[0]
+
+          body = JSON.parse(response[2][0])
+          assert_equal "Session not found", body["error"]
+        end
+
+        test "DELETE returns 404 after session is already deleted" do
+          init_request = create_rack_request(
+            "POST",
+            "/",
+            { "CONTENT_TYPE" => "application/json" },
+            { jsonrpc: "2.0", method: "initialize", id: "init" }.to_json,
+          )
+          init_response = @transport.handle_request(init_request)
+          session_id = init_response[1]["Mcp-Session-Id"]
+
+          first_delete = create_rack_request(
+            "DELETE",
+            "/",
+            { "HTTP_MCP_SESSION_ID" => session_id },
+          )
+          response = @transport.handle_request(first_delete)
+          assert_equal 200, response[0]
+
+          second_delete = create_rack_request(
+            "DELETE",
+            "/",
+            { "HTTP_MCP_SESSION_ID" => session_id },
+          )
+          response = @transport.handle_request(second_delete)
+          assert_equal 404, response[0]
+
+          body = JSON.parse(response[2][0])
+          assert_equal "Session not found", body["error"]
+        end
+
         test "handles DELETE request with missing session ID" do
           request = create_rack_request(
             "DELETE",