test: wire the handler property tests and make the proptest recipe run them

handler_proptest.rs was committed without ever being declared as a module, and it imported a list_size strategy that did not exist, so it never compiled. The proptest recipe filtered on a test name substring that matched no test.

Declare the module, add the two strategies it needs, and pass both property test filters to libtest so the recipe runs the ui and handler property tests.

justfile

@@ -49,7 +49,7 @@ doctest:
 # Run all proptests
 [group: 'test']
 proptest:
-    @cargo test proptest
+    @cargo test -- property_tests proptest
 
 alias test-snapshots := snapshot-tests
 # Run all snapshot tests