chore: import upstream snapshot with attribution

tests/test_multigraph_compat.py

@@ -0,0 +1,56 @@
+from __future__ import annotations
+
+import networkx as nx
+
+from graphify.multigraph_compat import (
+    CapabilityCheck,
+    MultigraphCapabilityResult,
+    probe_multigraph_capabilities,
+    require_multigraph_capabilities,
+)
+
+
+def test_probe_multigraph_capabilities_passes_current_runtime() -> None:
+    result = probe_multigraph_capabilities()
+
+    assert result.ok, result.error_message()
+    assert result.python_version
+    assert result.networkx_version
+    assert {check.name for check in result.checks} == {
+        "keyed_parallel_edges",
+        "node_link_edges_links_round_trip",
+        "duplicate_key_overwrite_semantics",
+        "reserved_key_attr_rejected",
+        "remove_edges_from_two_tuple_semantics",
+        "to_undirected_preserves_multigraph_type",
+    }
+
+
+def test_require_multigraph_capabilities_returns_result() -> None:
+    result = require_multigraph_capabilities()
+
+    assert result.ok
+
+
+def test_failure_message_is_actionable() -> None:
+    result = MultigraphCapabilityResult(
+        python_version="3.10.0",
+        networkx_version="0.0",
+        checks=(CapabilityCheck("node_link_edges_links_round_trip", False, "boom"),),
+    )
+
+    message = result.error_message()
+
+    assert "--multigraph requires NetworkX keyed MultiDiGraph node-link" in message
+    assert "Default simple graph mode remains available" in message
+    assert "node_link_edges_links_round_trip: boom" in message
+
+
+def test_networkx_duplicate_key_overwrite_trap_is_real() -> None:
+    graph = nx.MultiDiGraph()
+
+    graph.add_edge("a", "b", key="same", relation="first")
+    graph.add_edge("a", "b", key="same", relation="second")
+
+    assert graph.number_of_edges("a", "b") == 1
+    assert graph["a"]["b"]["same"]["relation"] == "second"