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/s3err/s3api_errors.go

@@ -4,6 +4,8 @@ import (
 	"encoding/xml"
 	"fmt"
 	"net/http"
+
+	"github.com/seaweedfs/seaweedfs/weed/util/constants"
 )
 
 // APIError structure
@@ -59,6 +61,7 @@ const (
 	ErrInvalidBucketName
 	ErrInvalidBucketState
 	ErrInvalidDigest
+	ErrBadDigest
 	ErrInvalidMaxKeys
 	ErrInvalidMaxUploads
 	ErrInvalidMaxParts
@@ -187,6 +190,11 @@ var errorCodeResponse = map[ErrorCode]APIError{
 		Description:    "The Content-Md5 you specified is not valid.",
 		HTTPStatusCode: http.StatusBadRequest,
 	},
+	ErrBadDigest: {
+		Code:           "BadDigest",
+		Description:    constants.ErrMsgBadDigest,
+		HTTPStatusCode: http.StatusBadRequest,
+	},
 	ErrInvalidMaxUploads: {
 		Code:           "InvalidArgument",
 		Description:    "Argument max-uploads must be an integer between 0 and 2147483647",