fix: return 500 for unreadable objects instead of 206

ErrObjectTampered was mapped to http.StatusPartialContent since upstream
ca6b4773e (2017), so a GET or HEAD of an object the server cannot decode
(invalid encrypted size, malformed actual-size, bad multipart ETag shape)
answered with a success status and an XML error document that SDKs handed
back as object content; boto3 returned the XML as Body and a zero-length
HEAD as success. Every origin of errObjectTampered is a stored-state
defect, not caller input, so map the entry to 500 Internal Server Error
and keep the XMinioObjectTampered code and message. The comment records
the deliberate divergence from upstream.

Tests: TestObjectTamperedGETHEADStatus (signed GET and HEAD, ErasureSD and
Erasure) fails with 206 on main and passes with 500; TestAPIErrCode,
TestAPIErrCodeDefinition, TestAPIHeadObjectHandler,
TestAPIHeadObjectHandlerWithEncryption and TestAPIGetObjectHandler stay
green. Compatibility: only the status line of one MinIO-specific error
changes; clients now retry damaged-object reads per their 5xx policy.

Fixes pgsty/silo#110

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01L7qJqWwy8oFA6aCXWRzXQe
Signed-off-by: Feng Ruohang <rh@vonng.com>

cmd/api-errors.go

@@ -1523,10 +1523,14 @@ var errorCodes = errorCodeMap{
 		Description:    "Your Host header is malformed.",
 		HTTPStatusCode: http.StatusBadRequest,
 	},
+	// The stored object cannot be served: a server-side data condition, not a
+	// successful partial read. Upstream maps it to http.StatusPartialContent
+	// (since ca6b4773e, 2017), which lets SDKs accept the XML error document
+	// as object content; SILO deliberately diverges and returns 500.
 	ErrObjectTampered: {
 		Code:           "XMinioObjectTampered",
 		Description:    errObjectTampered.Error(),
-		HTTPStatusCode: http.StatusPartialContent,
+		HTTPStatusCode: http.StatusInternalServerError,
 	},
 
 	ErrSiteReplicationInvalidRequest: {