libstore: Reallow unbracketed IPv6 addresses in store references

This implements a special back-compat shim to specifically allow
unbracketed IPv6 addresses in store references. This is something
that is relied upon in the wild and the old parsing logic accepted
both ways (brackets were optional). This patch restores this behavior.
As always, we didn't have any tests for this.

Addresses #13937.

src/libstore-tests/store-reference.cc

@@ -148,4 +148,39 @@ URI_TEST(
         .params = {},
     }))
 
+static StoreReference sshLoopbackIPv6{
+    .variant =
+        StoreReference::Specified{
+            .scheme = "ssh",
+            .authority = "[::1]",
+        },
+};
+
+URI_TEST_READ(ssh_unbracketed_ipv6_1, sshLoopbackIPv6)
+
+static StoreReference sshIPv6AuthorityWithUserinfo{
+    .variant =
+        StoreReference::Specified{
+            .scheme = "ssh",
+            .authority = "userinfo@[fea5:23e1:3916:fc24:cb52:2837:2ecb:ea8e]",
+        },
+};
+
+URI_TEST_READ(ssh_unbracketed_ipv6_2, sshIPv6AuthorityWithUserinfo)
+
+static StoreReference sshIPv6AuthorityWithUserinfoAndParams{
+    .variant =
+        StoreReference::Specified{
+            .scheme = "ssh",
+            .authority = "userinfo@[fea5:23e1:3916:fc24:cb52:2837:2ecb:ea8e]",
+        },
+    .params =
+        {
+            {"a", "b"},
+            {"c", "d"},
+        },
+};
+
+URI_TEST_READ(ssh_unbracketed_ipv6_3, sshIPv6AuthorityWithUserinfoAndParams)
+
 } // namespace nix