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/shell/command_s3_user_disable.go

@@ -2,6 +2,7 @@ package shell
 
 import (
 	"context"
+	"encoding/json"
 	"flag"
 	"fmt"
 	"io"
@@ -62,7 +63,6 @@ func (c *commandS3UserDisable) Do(args []string, commandEnv *CommandEnv, writer
 		}
 
 		if resp.Identity.Disabled {
-			fmt.Fprintf(writer, "User %q is already disabled.\n", *name)
 			return nil
 		}
 
@@ -77,6 +77,5 @@ func (c *commandS3UserDisable) Do(args []string, commandEnv *CommandEnv, writer
 		return err
 	}
 
-	fmt.Fprintf(writer, "Disabled user %q\n", *name)
-	return nil
+	return json.NewEncoder(writer).Encode(map[string]string{"name": *name, "status": "disabled"})
 }