feat(shell): add group management commands (#8993)

* feat(shell): add group management commands

Add weed shell commands for IAM group management:
- s3.group.create -name <group>
- s3.group.delete -name <group>
- s3.group.list
- s3.group.show -name <group>
- s3.group.add-user -group <group> -user <user>
- s3.group.remove-user -group <group> -user <user>

All commands use GetConfiguration/PutConfiguration gRPC pattern,
consistent with existing shell commands like s3.user.list.

* fix: add nil check for Configuration in group shell commands

Guard against nil Configuration response from GetConfiguration
gRPC call to prevent potential panics. (Gemini review)

weed/shell/command_s3_group_create.go

@@ -0,0 +1,79 @@
+package shell
+
+import (
+	"context"
+	"encoding/json"
+	"flag"
+	"fmt"
+	"io"
+	"time"
+
+	"github.com/seaweedfs/seaweedfs/weed/pb"
+	"github.com/seaweedfs/seaweedfs/weed/pb/iam_pb"
+	"google.golang.org/grpc"
+)
+
+func init() {
+	Commands = append(Commands, &commandS3GroupCreate{})
+}
+
+type commandS3GroupCreate struct {
+}
+
+func (c *commandS3GroupCreate) Name() string {
+	return "s3.group.create"
+}
+
+func (c *commandS3GroupCreate) Help() string {
+	return `create an S3 IAM group
+
+	s3.group.create -name <groupname>
+
+	Creates a new empty group. Add users with s3.group.add-user and
+	attach policies with s3.policy.attach or the IAM API.
+`
+}
+
+func (c *commandS3GroupCreate) HasTag(CommandTag) bool {
+	return false
+}
+
+func (c *commandS3GroupCreate) Do(args []string, commandEnv *CommandEnv, writer io.Writer) error {
+	f := flag.NewFlagSet(c.Name(), flag.ContinueOnError)
+	name := f.String("name", "", "group name")
+	if err := f.Parse(args); err != nil {
+		return err
+	}
+	if *name == "" {
+		return fmt.Errorf("-name is required")
+	}
+
+	return pb.WithGrpcClient(false, 0, func(conn *grpc.ClientConn) error {
+		client := iam_pb.NewSeaweedIdentityAccessManagementClient(conn)
+		ctx, cancel := context.WithTimeout(context.Background(), 30*time.Second)
+		defer cancel()
+
+		resp, err := client.GetConfiguration(ctx, &iam_pb.GetConfigurationRequest{})
+		if err != nil {
+			return err
+		}
+		cfg := resp.GetConfiguration()
+		if cfg == nil {
+			return fmt.Errorf("no IAM configuration found")
+		}
+
+		for _, g := range cfg.Groups {
+			if g.Name == *name {
+				return fmt.Errorf("group %s already exists", *name)
+			}
+		}
+
+		cfg.Groups = append(cfg.Groups, &iam_pb.Group{Name: *name})
+
+		if _, err := client.PutConfiguration(ctx, &iam_pb.PutConfigurationRequest{Configuration: cfg}); err != nil {
+			return err
+		}
+
+		return json.NewEncoder(writer).Encode(map[string]string{"group": *name})
+	}, commandEnv.option.FilerAddress.ToGrpcAddress(), false, commandEnv.option.GrpcDialOption)
+}