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_test.go

@@ -0,0 +1,46 @@
+package s3api
+
+import (
+	"testing"
+
+	"github.com/seaweedfs/seaweedfs/weed/s3api/s3err"
+	"github.com/seaweedfs/seaweedfs/weed/util/constants"
+)
+
+func TestFilerErrorToS3Error(t *testing.T) {
+	tests := []struct {
+		name        string
+		errString   string
+		expectedErr s3err.ErrorCode
+	}{
+		{
+			name:        "MD5 mismatch error",
+			errString:   constants.ErrMsgBadDigest,
+			expectedErr: s3err.ErrBadDigest,
+		},
+		{
+			name:        "Directory exists error",
+			errString:   "existing /path/to/file is a directory",
+			expectedErr: s3err.ErrExistingObjectIsDirectory,
+		},
+		{
+			name:        "File exists error",
+			errString:   "/path/to/file is a file",
+			expectedErr: s3err.ErrExistingObjectIsFile,
+		},
+		{
+			name:        "Unknown error",
+			errString:   "some random error",
+			expectedErr: s3err.ErrInternalError,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			result := filerErrorToS3Error(tt.errString)
+			if result != tt.expectedErr {
+				t.Errorf("filerErrorToS3Error(%q) = %v, want %v", tt.errString, result, tt.expectedErr)
+			}
+		})
+	}
+}