Fix #7305: Return 400 BadDigest instead of 500 InternalError for MD5 mismatch (#7306)

When an S3 upload has a mismatched Content-MD5 header, SeaweedFS was
incorrectly returning a 500 Internal Server Error instead of the proper
400 Bad Request with error code BadDigest (per AWS S3 specification).

Changes:
- Created weed/util/constants/filer.go with error message constants
- Added ErrMsgBadDigest constant for MD5 mismatch errors
- Added ErrMsgOperationNotPermitted constant for WORM permission errors
- Added ErrBadDigest error code with proper 400 status code mapping
- Updated filerErrorToS3Error() to detect MD5 mismatch and return ErrBadDigest
- Updated filer autoChunk() to return 400 Bad Request for MD5 mismatch
- Refactored error handling to use switch statement for better readability
- Ordered error checks with exact matches first for better maintainability
- Updated all error handling to use centralized constants
- Added comprehensive unit tests

All error messages now use constants from a single location for better
maintainability and consistency. Constants placed in util package to avoid
architectural dependency issues.

Fixes #7305

weed/server/filer_server_handlers_write.go

@@ -15,6 +15,7 @@ import (
 	"github.com/seaweedfs/seaweedfs/weed/operation"
 	"github.com/seaweedfs/seaweedfs/weed/pb/filer_pb"
 	"github.com/seaweedfs/seaweedfs/weed/security"
+	"github.com/seaweedfs/seaweedfs/weed/util/constants"
 	"github.com/seaweedfs/seaweedfs/weed/stats"
 	"github.com/seaweedfs/seaweedfs/weed/storage/needle"
 	"github.com/seaweedfs/seaweedfs/weed/util"
@@ -168,7 +169,7 @@ func (fs *FilerServer) move(ctx context.Context, w http.ResponseWriter, r *http.
 		return
 	} else if wormEnforced {
 		// you cannot move a worm file or directory
-		err = fmt.Errorf("cannot move write-once entry from '%s' to '%s': operation not permitted", src, dst)
+		err = fmt.Errorf("cannot move write-once entry from '%s' to '%s': %s", src, dst, constants.ErrMsgOperationNotPermitted)
 		writeJsonError(w, r, http.StatusForbidden, err)
 		return
 	}
@@ -228,7 +229,7 @@ func (fs *FilerServer) DeleteHandler(w http.ResponseWriter, r *http.Request) {
 		writeJsonError(w, r, http.StatusInternalServerError, err)
 		return
 	} else if wormEnforced {
-		writeJsonError(w, r, http.StatusForbidden, errors.New("operation not permitted"))
+		writeJsonError(w, r, http.StatusForbidden, errors.New(constants.ErrMsgOperationNotPermitted))
 		return
 	}