Add periodic cleanup job for DecayMap (#8) (#158)

* Add periodic cleanup job for DecayMap

see https://github.com/TecharoHQ/anubis/issues/8

* Refactor: Improve DecayMap cleanup tests and add Len method

- Refactored DecayMap cleanup tests to use the new Len method
  for more precise assertions.
- Added a Len method to DecayMap to retrieve the number of
  entries.
- Simplified conditional checks in Get method.

* chore(changelog): add entry

* fix(tests): Use Impl.expire for decaymap cleanup

Signed-off-by: Jason Cameron <git@jasoncameron.dev>

---------

Signed-off-by: Jason Cameron <git@jasoncameron.dev>

decaymap/decaymap_test.go

@@ -29,3 +29,32 @@ func TestImpl(t *testing.T) {
 		t.Error("got value even though it was supposed to be expired")
 	}
 }
+
+func TestCleanup(t *testing.T) {
+	dm := New[string, string]()
+
+	dm.Set("test1", "hi1", 1*time.Second)
+	dm.Set("test2", "hi2", 2*time.Second)
+	dm.Set("test3", "hi3", 3*time.Second)
+
+	dm.expire("test1") // Force expire test1
+	dm.expire("test2") // Force expire test2
+
+	dm.Cleanup()
+
+	finalLen := dm.Len() // Get the length after cleanup
+
+	if finalLen != 1 { // "test3" should be the only one left
+		t.Errorf("Cleanup failed to remove expired entries. Expected length 1, got %d", finalLen)
+	}
+
+	if _, ok := dm.Get("test1"); ok { // Verify Get still behaves correctly after Cleanup
+		t.Error("test1 should not be found after cleanup")
+	}
+	if _, ok := dm.Get("test2"); ok {
+		t.Error("test2 should not be found after cleanup")
+	}
+	if val, ok := dm.Get("test3"); !ok || val != "hi3" {
+		t.Error("test3 should still be found after cleanup")
+	}
+}