🧪 Add missing error path test for resolve_bind_target with unroutable IP

Adds a test to cover the branch in `resolve_bind_target` where an explicit error is returned if `iface_for_ip` returns `None` for a valid IP. This covers an otherwise untested error path, bringing branch coverage to 100% for this code block.

Co-authored-by: Horizontal42 <75383519+Horizontal42@users.noreply.github.com>

tests/test_iface.py

@@ -61,6 +61,16 @@ def test_resolve_bind_target_by_exact_name():
     assert result.error is None
 
 
+def test_resolve_bind_target_by_unmapped_ip_is_an_error():
+    result = resolve_bind_target(
+        "10.255.255.255",
+        addrs(Ethernet=[(socket.AF_INET, "192.168.3.72")]),
+        up(Ethernet=True),
+    )
+    assert result.error is not None
+    assert result.error == "no adapter has the address '10.255.255.255'"
+
+
 def test_resolve_bind_target_by_ip_finds_owning_adapter():
     result = resolve_bind_target(
         "192.168.3.72",