filer: add structured error codes to CreateEntryResponse (#8767)

* filer: add FilerError enum and error_code field to CreateEntryResponse

Add a machine-readable error code alongside the existing string error
field. This follows the precedent set by PublishMessageResponse in the
MQ broker proto. The string field is kept for human readability and
backward compatibility.

Defined codes: OK, ENTRY_NAME_TOO_LONG, PARENT_IS_FILE,
EXISTING_IS_DIRECTORY, EXISTING_IS_FILE, ENTRY_ALREADY_EXISTS.

* filer: add sentinel errors and error code mapping in filer_pb

Define sentinel errors (ErrEntryNameTooLong, ErrParentIsFile, etc.) in
the filer_pb package so both the filer and consumers can reference them
without circular imports.

Add FilerErrorToSentinel() to map proto error codes to sentinels, and
update CreateEntryWithResponse() to check error_code first, falling back
to the string-based path for backward compatibility with old servers.

* filer: return wrapped sentinel errors and set proto error codes

Replace fmt.Errorf string errors in filer.CreateEntry, UpdateEntry, and
ensureParentDirectoryEntry with wrapped filer_pb sentinel errors (using
%w). This preserves errors.Is() traversal on the server side.

In the gRPC CreateEntry handler, map sentinel errors to the
corresponding FilerError proto codes using errors.Is(), setting both
resp.Error (string, for backward compat) and resp.ErrorCode (enum).

* S3: use errors.Is() with filer sentinels instead of string matching

Replace fragile string-based error matching in filerErrorToS3Error and
other S3 API consumers with errors.Is() checks against filer_pb sentinel
errors. This works because the updated CreateEntryWithResponse helper
reconstructs sentinel errors from the proto FilerError code.

Update iceberg stage_create and metadata_files to check resp.ErrorCode
instead of parsing resp.Error strings. Update SSE-S3 to use errors.Is()
for the already-exists check.

String matching is retained only for non-filer errors (gRPC transport
errors, checksum validation) that don't go through CreateEntryResponse.

* filer: remove backward-compat string fallbacks for error codes

Clients and servers are always deployed together, so there is no need
for backward-compatibility fallback paths that parse resp.Error strings
when resp.ErrorCode is unset. Simplify all consumers to rely solely on
the structured error code.

* iceberg: ensure unknown non-OK error codes are not silently ignored

When FilerErrorToSentinel returns nil for an unrecognized error code,
return an error including the code and message rather than falling
through to return nil.

* filer: fix redundant error message and restore error wrapping in helper

Use request path instead of resp.Error in the sentinel error format
string to avoid duplicating the sentinel message (e.g. "entry already
exists: entry already exists"). Restore %w wrapping with errors.New()
in the fallback paths so callers can use errors.Is()/errors.As().

* filer: promote file to directory on path conflict instead of erroring

S3 allows both "foo/bar" (object) and "foo/bar/xyzzy" (another object)
to coexist because S3 has a flat key space. When ensureParentDirectoryEntry
finds a parent path that is a file instead of a directory, promote it to
a directory by setting ModeDir while preserving the original content and
chunks. Use Store.UpdateEntry directly to bypass the Filer.UpdateEntry
type-change guard.

This fixes the S3 compatibility test failures where creating overlapping
keys (e.g. "foo/bar" then "foo/bar/xyzzy") returned ExistingObjectIsFile.

weed/pb/filer_pb/filer_pb_helper.go

@@ -145,6 +145,13 @@ func CreateEntryWithResponse(ctx context.Context, client SeaweedFilerClient, req
 		glog.V(1).InfofCtx(ctx, "create entry %s/%s %v: %v", request.Directory, request.Entry.Name, request.OExcl, err)
 		return nil, fmt.Errorf("CreateEntry: %w", err)
 	}
+	if resp.ErrorCode != FilerError_OK {
+		glog.V(1).InfofCtx(ctx, "create entry %s/%s %v: %v (code %v)", request.Directory, request.Entry.Name, request.OExcl, resp.Error, resp.ErrorCode)
+		if sentinel := FilerErrorToSentinel(resp.ErrorCode); sentinel != nil {
+			return nil, fmt.Errorf("CreateEntry %s/%s: %w", request.Directory, request.Entry.Name, sentinel)
+		}
+		return nil, fmt.Errorf("CreateEntry: %w", errors.New(resp.Error))
+	}
 	if resp.Error != "" {
 		glog.V(1).InfofCtx(ctx, "create entry %s/%s %v: %v", request.Directory, request.Entry.Name, request.OExcl, resp.Error)
 		return nil, fmt.Errorf("CreateEntry: %w", errors.New(resp.Error))
@@ -183,6 +190,37 @@ func LookupEntry(ctx context.Context, client SeaweedFilerClient, request *Lookup
 
 var ErrNotFound = errors.New("filer: no entry is found in filer store")
 
+// Sentinel errors for filer entry operations.
+// These are set by the filer and reconstructed from FilerError codes after
+// crossing the gRPC boundary, so consumers can use errors.Is() instead of
+// parsing error strings.
+var (
+	ErrEntryNameTooLong    = errors.New("entry name too long")
+	ErrParentIsFile        = errors.New("parent path is a file")
+	ErrExistingIsDirectory = errors.New("existing entry is a directory")
+	ErrExistingIsFile      = errors.New("existing entry is a file")
+	ErrEntryAlreadyExists  = errors.New("entry already exists")
+)
+
+// FilerErrorToSentinel maps a proto FilerError code to its sentinel error.
+// Returns nil for OK or unknown codes.
+func FilerErrorToSentinel(code FilerError) error {
+	switch code {
+	case FilerError_ENTRY_NAME_TOO_LONG:
+		return ErrEntryNameTooLong
+	case FilerError_PARENT_IS_FILE:
+		return ErrParentIsFile
+	case FilerError_EXISTING_IS_DIRECTORY:
+		return ErrExistingIsDirectory
+	case FilerError_EXISTING_IS_FILE:
+		return ErrExistingIsFile
+	case FilerError_ENTRY_ALREADY_EXISTS:
+		return ErrEntryAlreadyExists
+	default:
+		return nil
+	}
+}
+
 func IsEmpty(event *SubscribeMetadataResponse) bool {
 	return event.EventNotification.NewEntry == nil && event.EventNotification.OldEntry == nil
 }