Fix update_dataclass_from_json for None/frozen nested dataclasses

[richard-to]

_recursive_update_dataclass_from_json_obj rebuilt a nested dataclass field
by recursing into whatever the attribute currently held. That breaks for a
field typed X | None holding None (nothing to recurse into, raises
"Unhandled stateclass deserialization") and for a frozen nested dataclass
(setattr raises FrozenInstanceError). Both cases surface inside
update_state, before any event handler runs, so the interaction silently
does nothing.

Build nested dataclass values (including list items) directly from the
JSON payload via the constructor, resolving X | None to X from the
type hint, instead of instantiating empty and mutating with setattr.

Fixes #1401

Co-Authored-By: Claude Sonnet 5 noreply@anthropic.com
Claude-Session: https://claude.ai/code/session_019XKNGzaycAUrysh8HgHpPX

[richard-to]

The test check failure is a single Playwright e2e timeout in mesop/components/box/e2e/box_events_test.ts:18 ("test right click event"), waiting on getByText('Is Click: True'). This is unrelated to this PR's change (mesop/dataclass_utils/dataclass_utils.py and its test file only) — CI is green on the base commit (main @ 9ca3b56), and this test doesn't touch anything in the diff. It looks like a timing flake in an unrelated component.

I don't have permission to trigger a re-run from here (got a 403 on the rerun-failed-jobs API). Flagging so a maintainer can re-run the job; I'll keep watching this PR for further CI/review activity.

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