shell: refuse s3.bucket.create on an existing bucket (#11455)

* shell: refuse s3.bucket.create on an existing bucket

CreateEntry without o_excl replaces the bucket entry, dropping every
extended attribute: lifecycle configuration, owner, versioning and the
irreversible Object Lock flag. Send o_excl so a re-run fails with
'bucket already exists' instead of silently resetting the bucket.

* filer: fail exclusive creates when the lookup itself fails

CreateEntry discards FindEntry errors, so an o_excl create hitting a
transient store failure would take the insert path and upsert over the
entry it was meant to preserve. Propagate the lookup error when o_excl
is set; non-exclusive creates keep their existing semantics.

Generated with [Devin](https://devin.ai)

Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

* shell: test s3.bucket.create requests an exclusive create

Exercises the command end to end through a fake filer gRPC server and
asserts the OExcl flag reaches the wire along with the already-exists
error path.

Generated with [Devin](https://devin.ai)

Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

* shell: synchronize captured requests and assert the exact bucket error

The fake filer records CreateEntry requests on the gRPC server goroutine,
so reads need the same mutex; the test also now checks for the exact
"bucket my-bucket already exists" message rather than any error that
mentions existence.

Generated with [Devin](https://devin.ai)

Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

---------

Co-authored-by: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

weed/shell/command_s3_bucket_create.go

@@ -2,6 +2,7 @@ package shell
 
 import (
 	"context"
+	"errors"
 	"flag"
 	"fmt"
 	"io"
@@ -112,11 +113,16 @@ func (c *commandS3BucketCreate) Do(args []string, commandEnv *CommandEnv, writer
 			entry.Extended[s3_constants.ExtObjectLockEnabledKey] = []byte(s3_constants.ObjectLockEnabled)
 		}
 
-		if _, err := client.CreateEntry(context.Background(), &filer_pb.CreateEntryRequest{
+		createErr := filer_pb.CreateEntry(context.Background(), client, &filer_pb.CreateEntryRequest{
 			Directory: filerBucketsPath,
 			Entry:     entry,
-		}); err != nil {
-			return err
+			OExcl:     true,
+		})
+		if errors.Is(createErr, filer_pb.ErrEntryAlreadyExists) {
+			return fmt.Errorf("bucket %s already exists", *bucketName)
+		}
+		if createErr != nil {
+			return createErr
 		}
 
 		fmt.Fprintln(writer, "created bucket", *bucketName)