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

1 files changed, 7 insertions, 0 deletions

diff --git a/weed/util/constants/filer.go b/weed/util/constants/filer.go
new file mode 100644
index 000000000..f5f240e76
--- /dev/null
+++ b/weed/util/constants/filer.go
@@ -0,0 +1,7 @@
+package constants
+
+// Filer error messages
+const (
+	ErrMsgOperationNotPermitted = "operation not permitted"
+	ErrMsgBadDigest             = "The Content-Md5 you specified did not match what we received."
+)