fix: Rust-server bytes response fixed to not attempt string conversion

[dsteeley]

Fix bug in rust-server where it was attempting to convert bytes to string and failing on invalid UTF-8 format. Fixed this up in multiple locations and the multipart support.

Error hit - Error converting octet stream to string: FromUtf8Error { bytes: [...], error: Utf8Error { valid_up_to: 1, error_len: Some(1) }
Endpoint this was hit on

     content:
        application/gzip:
          schema:
            type: string
            format: binary

Resolve by directly returning the bytes rather than attempting string conversion.

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[wing328]

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