fix(s3api): handle directory markers with custom Content-Type headers

Fixes #8712: S3 API directory markers created via PutObject with an
explicit Content-Type header (e.g. application/octet-stream) were not
appearing in ListObjects results.

Root cause: The filer strips "application/octet-stream" from the Mime
field during InsertEntry/UpdateEntry (filerstore_wrapper.go). This
causes IsDirectoryKeyObject() to return false, so the directory markers
are excluded from listings.

Additionally, when AWS CLI includes a Content-Type header, it may use
AWS chunked encoding. r.ContentLength then includes chunked overhead,
which can exceed 1024 bytes even for zero-byte payloads, preventing the
directory marker fast-path from triggering.

Changes:
- Use x-amz-decoded-content-length header for accurate payload size
- Require actualContentLength >= 0 to reject unknown/negative lengths
- Wrap body reader with io.LimitReader(1024+1) to prevent OOM
- Reject payloads > 1024 bytes with HTTP 413
- Store MIME in ExtMimeType extended attribute for zero-byte markers
  (persists even when the filer strips the Mime field)
- Update IsDirectoryKeyObject() to check ExtMimeType as fallback

weed/s3api/s3_constants/extend_key.go

@@ -19,6 +19,7 @@ const (
 	ExtLatestVersionOwnerKey       = "Seaweed-X-Amz-Latest-Version-Owner"
 	ExtLatestVersionIsDeleteMarker = "Seaweed-X-Amz-Latest-Version-Is-Delete-Marker"
 	ExtMultipartObjectKey          = "key"
+	ExtMimeType                    = "Seaweed-X-Amz-Mime-Type"
 
 	// Bucket Policy
 	ExtBucketPolicyKey = "Seaweed-X-Amz-Bucket-Policy"