shell: s3.* commands always output JSON, connection messages to stderr (#8976)

* shell: s3.* commands output JSON, connection messages to stderr

All s3.user.* and s3.policy.attach|detach commands now output structured
JSON to stdout instead of human-readable text:

- s3.user.create: {"name","access_key"} (secret key to stderr only)
- s3.user.list: [{name,status,policies,keys}]
- s3.user.show: {name,status,source,account,policies,credentials,...}
- s3.user.delete: {"name"}
- s3.user.enable/disable: {"name","status"}
- s3.policy.attach/detach: {"policy","user"}

Connection startup messages (master/filer) moved to stderr so they
don't pollute structured output when piping.

Closes #8962 (partial — covers merged s3.user/policy commands).

* shell: fix secret leak, duplicate JSON output, and non-interactive prompt

- s3.user.create: only echo secret key to stderr when auto-generated,
  never echo caller-supplied secrets
- s3.user.enable/disable: fix duplicate JSON output — remove inner
  write in early-return path, keep single write site after gRPC call
- shell_liner: use bufio.Scanner when stdin is not a terminal instead
  of liner.Prompt, suppressing the "> " prompt in piped mode

* shell: check scanner error, idempotent enable output, history errors to stderr

- Check scanner.Err() after non-interactive input loop to surface read errors
- s3.user.enable: always emit JSON regardless of current state (idempotent)
- saveHistory: write error messages to stderr instead of stdout

weed/command/shell.go

@@ -2,6 +2,7 @@ package command
 
 import (
 	"fmt"
+	"os"
 
 	"github.com/seaweedfs/seaweedfs/weed/pb"
 
@@ -60,7 +61,7 @@ func runShell(command *Command, args []string) bool {
 		filerAddress = viper.GetString("cluster." + cluster + ".filer")
 	}
 	shellOptions.FilerAddress = pb.ServerAddress(filerAddress)
-	fmt.Printf("master: %s filer: %s\n", *shellOptions.Masters, shellOptions.FilerAddress)
+	fmt.Fprintf(os.Stderr, "master: %s filer: %s\n", *shellOptions.Masters, shellOptions.FilerAddress)
 
 	shell.RunShell(shellOptions)