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/s3api/s3api_object_handlers_put.go

@@ -20,6 +20,7 @@ import (
 	"github.com/seaweedfs/seaweedfs/weed/s3api/s3err"
 	"github.com/seaweedfs/seaweedfs/weed/security"
 	weed_server "github.com/seaweedfs/seaweedfs/weed/server"
+	"github.com/seaweedfs/seaweedfs/weed/util/constants"
 	stats_collect "github.com/seaweedfs/seaweedfs/weed/stats"
 )
 
@@ -380,6 +381,8 @@ func setEtag(w http.ResponseWriter, etag string) {
 
 func filerErrorToS3Error(errString string) s3err.ErrorCode {
 	switch {
+	case errString == constants.ErrMsgBadDigest:
+		return s3err.ErrBadDigest
 	case strings.HasPrefix(errString, "existing ") && strings.HasSuffix(errString, "is a directory"):
 		return s3err.ErrExistingObjectIsDirectory
 	case strings.HasSuffix(errString, "is a file"):