dsh-image-base64: Fixing 400 Errors for WebP Images in LM Studio and Other Gateways

`dsh-image-base64` is a plugin for DeepSeek Harness (DSH) designed to resolve 400 errors caused by incompatible image formats in gateways such as LM Studio. By default, DSH's attachment plugin converts images to `image/webp`, while gateways typically only accept PNG, JPEG, or GIF. This plugin uses the `sharp` library to intercept images before a request is sent and transcode unsupported formats to PNG, thereby eliminating protocol-level mismatches. It only modifies request data without affecting storage format, though it may increase file size.