fix: use correct default URL constant for netgroup_url

[Nepomuk5665]

Summary

Fix a copy-paste bug in HttpFilesSource._SetDefaults() where netgroup_url incorrectly defaults to GROUP_URL instead of NETGROUP_URL.

The Bug

In nss_cache/sources/httpsource.py line 94, the default value for netgroup_url was set to self.GROUP_URL:

if "netgroup_url" not in configuration:
    configuration["netgroup_url"] = self.GROUP_URL  # Wrong!

This is clearly a copy-paste error from the group_url handling on line 91-92 directly above it.

The Fix

Changed to use the correct constant self.NETGROUP_URL:

if "netgroup_url" not in configuration:
    configuration["netgroup_url"] = self.NETGROUP_URL  # Correct

Impact

Without this fix, any netgroup lookups using the HTTP source that don't explicitly configure netgroup_url would incorrectly use the group URL by default, leading to failed or incorrect netgroup resolution.