starlark: make Starlark.addMethods reject fields

Previously, addMethods would also create a BuiltinCallable for
each Java method annotated with structField=True, even though
it was never intended to support that. Field methods are
problematic because merely accessing them invokes the Java
function, yet we don't have the correct Semantics available,
nor can we possibly deal with an InterruptedException.
Now, it rejects them.

- Add test of addMethods, and assertion in BuiltinCallable.
- Update tests of "proxy object" to use dir+getattr instead
  of addMethods.
- Improve doc comments.
- un-deprecate getAnnotation method. It seems reasonable given
  that the annotations are public API, and it is related to the
  core purpose of the class.
- change string method to report "<built-in method append of list value>"
  when appropriate, matching Python[23] and go.starlark.net.
  This is a minor incompatible behavior change, but I could not find
  any places in Google's code base that would break, which is a good sign.
- Replace calls to private BuiltinCallable() in tests with getattr.

PiperOrigin-RevId: 332477998