Improve handling of SSH_FXP_FSETSTAT requests during ICAP antivirus scanning

When ICAP antivirus scanning is enabled for SFTP transfers, SSP may receive additional valid SFTP requests while a file is still being processed. One example is the SSH_FXP_FSETSTAT request, which some SFTP clients send during an active upload.

In the current behavior, if ICAP scanning is in progress, SSP does not return any response to the SSH_FXP_FSETSTAT request. As a result, the client waits until its timeout expires, causing the file transfer or automation process to stall or fail.

When ICAP scanning is disabled, the same request receives an appropriate SFTP status response, allowing the client to continue processing according to the protocol.

The current recommendation is for clients to avoid sending SSH_FXP_FSETSTAT during file upload and instead change their upload sequence. However, many existing SFTP clients cannot modify their protocol behavior, and interoperability should not depend on client-specific implementations.

The request is not to make SSH_FXP_FSETSTAT succeed during ICAP scanning. Rather, SSP should ensure that valid SFTP requests are not left without a protocol-compliant response. How such requests are handled during ICAP processing is an implementation detail, but the client should ultimately receive a definitive outcome instead of waiting until a network timeout occurs.