Move structural typing tests into interface unit test files.

Align test layout with the one-test-file-per-interface convention by removing structural_typing_test.py.

Co-authored-by: Cursor <[email protected]>

tests/unit/json_repository_interface_test.py

@@ -1,5 +1,6 @@
 """Unit tests for JsonRepositoryInterface."""
 
+from collections.abc import Iterator
 from typing import Any
 
 import pytest
@@ -9,6 +10,36 @@ from python_repositories.interfaces.json_repository_interface import (
 )
 
 
+class InMemoryJsonRepo:
+    """Plain class that satisfies JsonRepositoryInterface without inheritance."""
+
+    def get(self, key: str) -> dict[str, Any] | None:
+        return None
+
+    def set(self, key: str, data: dict[str, Any]) -> None:
+        pass
+
+    def delete(self, key: str) -> None:
+        pass
+
+    def list_keys(self, pattern: str) -> list[str]:
+        return []
+
+    def scan_keys(
+        self,
+        pattern: str,
+        *,
+        count: int | None = None,
+    ) -> Iterator[str]:
+        del count
+        yield from self.list_keys(pattern)
+
+
+def accepts_json_repo(repo: JsonRepositoryInterface) -> None:
+    """Type-checking hook for JsonRepositoryInterface structural subtyping."""
+    repo.get("key")
+
+
 def test_instantiation_fails_when_get_not_implemented() -> None:
     """Test that instantiation fails if get is not implemented."""
 
@@ -104,3 +135,10 @@ def test_scan_keys_defaults_to_list_keys() -> None:
     repository = Complete()
 
     assert list(repository.scan_keys("user")) == ["user-1", "user-2"]
+
+
+def test_structural_subtyping() -> None:
+    """Test that a plain class satisfies JsonRepositoryInterface structurally."""
+    repo: JsonRepositoryInterface = InMemoryJsonRepo()
+    accepts_json_repo(repo)
+    assert isinstance(repo, JsonRepositoryInterface)