Align error wording with repo convention

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: SamMorrowDrums

pkg/github/server.go

@@ -205,7 +205,7 @@ func NewServer(version, name, title string, opts *mcp.ServerOptions) *mcp.Server
 func CompletionsHandler(getClient GetClientFn) func(ctx context.Context, req *mcp.CompleteRequest) (*mcp.CompleteResult, error) {
 	return func(ctx context.Context, req *mcp.CompleteRequest) (*mcp.CompleteResult, error) {
 		if req == nil || req.Params == nil || req.Params.Ref == nil {
-			return nil, fmt.Errorf("invalid request: missing ref parameter")
+			return nil, fmt.Errorf("missing required parameter: ref")
 		}
 		switch req.Params.Ref.Type {
 		case "ref/resource":

pkg/github/server_test.go

@@ -369,7 +369,7 @@ func TestCompletionsHandler_RejectsMissingRef(t *testing.T) {
 			result, err := handler(context.Background(), tc.req)
 			require.Error(t, err)
 			assert.Nil(t, result)
-			assert.Contains(t, err.Error(), "missing ref parameter")
+			assert.Contains(t, err.Error(), "missing required parameter: ref")
 		})
 	}
 }