test(core): add unit tests for untested core modules

Adds 113 tests covering:
- abc: RegistryABC (concrete implementation + edge cases)
- environment: Environment get/set, EnvironmentEnum
- pipes: BoolPipe, IPAddressPipe (incl. roundtrip + error cases)
- time: Cron (next(), intervals, invalid expression)
- utils: Cache (TTL, expiry, cleanup), get_value (incl. bug
  documentation: cast result not returned for string->typed values),
  JSONProcessor (nested objects, enums, defaults)
- property: classproperty (class access, instance access, subclass)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

test/core/abc/registry_abc_test.py

@@ -0,0 +1,66 @@
+import pytest
+from cpl.core.abc.registry_abc import RegistryABC
+
+
+class StringRegistry(RegistryABC[str]):
+    def __init__(self):
+        super().__init__()
+
+    def extend(self, items: list[str]) -> None:
+        for item in items:
+            self.add(item)
+
+    def add(self, item: str) -> None:
+        self._items[item] = item
+
+    def get(self, key: str) -> str | None:
+        return self._items.get(key)
+
+    def all(self) -> list[str]:
+        return list(self._items.values())
+
+
+def test_add_and_get():
+    reg = StringRegistry()
+    reg.add("hello")
+    assert reg.get("hello") == "hello"
+
+
+def test_get_missing_returns_none():
+    reg = StringRegistry()
+    assert reg.get("nonexistent") is None
+
+
+def test_all_empty():
+    reg = StringRegistry()
+    assert reg.all() == []
+
+
+def test_all_returns_all_items():
+    reg = StringRegistry()
+    reg.add("a")
+    reg.add("b")
+    reg.add("c")
+    items = reg.all()
+    assert set(items) == {"a", "b", "c"}
+
+
+def test_extend():
+    reg = StringRegistry()
+    reg.extend(["x", "y", "z"])
+    assert reg.get("x") == "x"
+    assert reg.get("y") == "y"
+    assert reg.get("z") == "z"
+    assert len(reg.all()) == 3
+
+
+def test_overwrite_key():
+    reg = StringRegistry()
+    reg.add("key")
+    reg.add("key")
+    assert len(reg.all()) == 1
+
+
+def test_cannot_instantiate_abc_directly():
+    with pytest.raises(TypeError):
+        RegistryABC()