util.py: revert leading zeros logic, add is_valid_pfx

the previously added check for leading zeros picks up private networks,
so it's not exactly what we want here. Instead, separate the format_pfx
and is_valid_pfx intents and add tests accordingly.
format_pfx checks whether the existing prefix can be coerced to a string
and returns it. is_valid_pfx simply checks whether it's a valid pfx
or not.

kartograf/util.py

@@ -133,22 +133,33 @@ def wait_for_launch(wait):
 
 def format_pfx(pfx):
     """
-    We have seen some formatting issues like leading zeros in the prefix,
-    which can cause problems.
+    Attempt to format an IP network or address.
+    If invalid, return the input unchanged.
     """
     try:
         if "/" in pfx:
-            pattern = r"^0+"
-            match = re.search(pattern, pfx)
-            if match:
-                pfx = re.sub(pattern, "", pfx)
             formatted_pfx = str(ipaddress.ip_network(pfx))
             return f"{formatted_pfx}"
         return str(ipaddress.ip_address(pfx))
     except ValueError:
         return pfx
 
 
+def is_valid_pfx(pfx):
+    """
+    Check whether the IP network or address provided is valid.
+    """
+    try:
+        if "/" in pfx:
+            ipaddress.ip_network(pfx)
+            return True
+        else:
+            ipaddress.ip_address(pfx)
+            return True
+    except ValueError:
+        return False
+
+
 def get_root_network(pfx):
     """
     Extract the top-level network from an IPv4 or IPv6 address.

tests/util_test.py

@@ -1,4 +1,4 @@
-from kartograf.util import format_pfx, get_root_network
+from kartograf.util import format_pfx, is_valid_pfx, get_root_network
 
 def test_valid_ipv4_network():
     pfx = "192.144.11.0/21"
@@ -20,18 +20,6 @@ def test_valid_ipv6_addr():
     assert format_pfx(pfx) == pfx
 
 
-def test_ipv4_prefix_with_leading_zeros():
-    pfx = "010.10.00.00/16"
-    expected_output = "10.10.00.00/16"
-    assert format_pfx(pfx) == expected_output
-
-
-def test_ipv6_prefix_with_leading_zeros():
-    pfx = "001:db8::0/24"
-    expected_output = "1:db8::0/24"
-    assert format_pfx(pfx) == expected_output
-
-
 def test_invalid_ip_network():
     pfx = "192.1/asdf"
     assert format_pfx(pfx) == pfx
@@ -42,6 +30,18 @@ def test_invalid_input():
     assert format_pfx(pfx) == pfx
 
 
+def test_ipv4_prefix_with_leading_zeros():
+    pfx = "010.10.00.00/16"
+    assert format_pfx(pfx) == pfx
+    assert not is_valid_pfx(pfx)
+
+
+def test_ipv6_prefix_with_leading_zeros():
+    pfx = "001:db8::0/24"
+    assert format_pfx(pfx) == pfx
+    assert not is_valid_pfx(pfx)
+
+
 def test_get_root_network():
     ipv4 = "192.144.11.0/24"
     assert get_root_network(ipv4) == 192