test(integration): cover fsconfig/fspick/open_tree in mount-API scenario

Extend the mountfs-management scenario with best-effort fsconfig (KindFd),
fspick (KindPathname), and open_tree (KindOpen) calls to complete
new-mount-API end-to-end coverage. fsconfig reuses the fscontext fd from
fsopen (FSCONFIG_SET_STRING + FSCONFIG_CMD_CREATE), fspick targets "/"
with FSPICK_NO_AUTOMOUNT, and open_tree clones the scenario mount point
with OPEN_TREE_CLONE|OPEN_TREE_CLOEXEC. All returned fds are closed and
all errno values are ignored, so ENOSYS/EPERM/EINVAL/EBADF are tolerated;
the sys_enter_ tracepoints fire on kernel entry regardless, creating no
mounts on the host.

Assert enter_fsconfig/enter_fspick/enter_open_tree (MinCount>=1) in
TestMountFsManagementSyscalls and add the three syscalls to the trace
filter. Gating is unchanged (root-only via the shared harness).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

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