refactor(http): clarify subscription rejection

Document why header validation precedes the unsupported-method rejection and use named SDK error constants in tests.

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>
Copilot-Session: 06d5dda1-4086-4996-8d18-152e45e611b0

Author: SamMorrowDrums

pkg/http/handler.go

@@ -223,6 +223,8 @@ func (h *Handler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
+	// Let the SDK validate missing or mismatched method headers before the
+	// middleware rejects a well-formed request as unsupported.
 	if r.Header.Get(headers.MCPMethodHeader) == subscriptionsListenMethod {
 		ghServer.AddReceivingMiddleware(rejectSubscriptionsListen)
 	}

pkg/http/handler_test.go

@@ -19,6 +19,7 @@ import (
 	"github.com/github/github-mcp-server/pkg/translations"
 	"github.com/github/github-mcp-server/pkg/utils"
 	"github.com/go-chi/chi/v5"
+	"github.com/modelcontextprotocol/go-sdk/jsonrpc"
 	"github.com/modelcontextprotocol/go-sdk/mcp"
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
@@ -937,18 +938,18 @@ func TestSubscriptionsListenIsRejected(t *testing.T) {
 			name:             "matching method header",
 			methodHeader:     subscriptionsListenMethod,
 			expectedStatus:   http.StatusNotFound,
-			expectedJSONCode: -32601,
+			expectedJSONCode: jsonrpc.CodeMethodNotFound,
 		},
 		{
 			name:             "missing method header",
 			expectedStatus:   http.StatusBadRequest,
-			expectedJSONCode: -32020,
+			expectedJSONCode: mcp.CodeHeaderMismatch,
 		},
 		{
 			name:             "mismatched method header",
 			methodHeader:     "tools/list",
 			expectedStatus:   http.StatusBadRequest,
-			expectedJSONCode: -32020,
+			expectedJSONCode: mcp.CodeHeaderMismatch,
 		},
 	}