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/time/cron_test.py

@@ -0,0 +1,60 @@
+import pytest
+from datetime import datetime
+from cpl.core.time.cron import Cron
+
+
+def test_next_returns_datetime():
+    cron = Cron("* * * * *")
+    result = cron.next()
+    assert isinstance(result, datetime)
+
+
+def test_next_is_in_the_future():
+    cron = Cron("* * * * *")
+    result = cron.next()
+    assert result > datetime.now()
+
+
+def test_next_called_multiple_times_is_monotonic():
+    cron = Cron("* * * * *")
+    results = [cron.next() for _ in range(5)]
+    for i in range(1, len(results)):
+        assert results[i] > results[i - 1]
+
+
+def test_every_minute_interval():
+    start = datetime(2024, 1, 1, 12, 0, 0)
+    cron = Cron("* * * * *", start_time=start)
+    first = cron.next()
+    second = cron.next()
+    diff = (second - first).total_seconds()
+    assert diff == 60
+
+
+def test_hourly_interval():
+    start = datetime(2024, 1, 1, 12, 0, 0)
+    cron = Cron("0 * * * *", start_time=start)
+    first = cron.next()
+    second = cron.next()
+    diff = (second - first).total_seconds()
+    assert diff == 3600
+
+
+def test_daily_midnight():
+    start = datetime(2024, 1, 1, 0, 0, 0)
+    cron = Cron("0 0 * * *", start_time=start)
+    first = cron.next()
+    assert first.hour == 0
+    assert first.minute == 0
+
+
+def test_custom_start_time():
+    start = datetime(2024, 6, 15, 10, 30, 0)
+    cron = Cron("*/5 * * * *", start_time=start)
+    result = cron.next()
+    assert result > start
+
+
+def test_invalid_cron_expression():
+    with pytest.raises(Exception):
+        Cron("invalid expression")