RESPONDER: fix responder_set_fd_limit()

[alexey-tikhonov]

to not even try setting hard limit as SSSD never has CAP_SYS_RESOURCE

[gemini-code-assist]

Code Review

This pull request refactors responder_set_fd_limit() to avoid attempting to set the hard limit for file descriptors, as SSSD responders typically lack the necessary CAP_SYS_RESOURCE capability. The change correctly removes the initial setrlimit() call that was likely to fail or behave incorrectly, and instead directly queries the current hard limit to safely adjust the soft limit. The corresponding documentation in sssd.conf.5.xml has been updated to reflect this behavior, and a helpful debug message has been added for cases where the requested limit exceeds the hard limit. The changes are correct, improve efficiency by removing an unnecessary system call, and make the code's intent clearer. I have no further suggestions.

[alexey-tikhonov]

Note: Covscan is green.