* GH-3247: Fix `SftpSession.exists` for error code
Fixes https://github.com/spring-projects/spring-integration/issues/3247
When there is no path on the SFTP server, a `ChannelSftp.SSH_FX_NO_SUCH_FILE`
error is returned in the thrown `SftpException`.
* Fix `SftpSession.exists()` to check for the `SSH_FX_NO_SUCH_FILE` to
return `false` and re-throw an exception otherwise
* Add mock test for `SftpSession.exists()`
* Add `org.mockito.AdditionalMatchers` to `checkstyle.xml` exclusions
**Cherry-pick to 5.2.x & 5.1.x**
* * Add exists tests against Mina embedded server
b75f1bd0da