Add @Nullable to collection and map type arguments that hold null elements

Places the JSpecify @Nullable annotation on a collection or map type argument when its elements or values are provably nullable, producing List<@Nullable String> or Map<String, @Nullable String> (the list may hold null elements) rather than @Nullable List<String> (the list reference may be null), which NullAway enforces in JSpecify mode. A type argument is annotated when Collection.add(...) is called with a provably-null element, Map.put(...) with a provably-null value, or the declaration's initializer is a List.of(...)/Set.of(...)/Arrays.asList(...) containing a null. The receiver is resolved to its declaration within the same compilation unit (cross-file declarations are not handled). Conservative by design: it skips wildcard and raw type arguments and ones already @Nullable.