[codex] fix component load event target

[thomwolf]

Summary

• keep component-attached load events targeted at the root load event instead of serializing a null target
• add regression coverage for component load-event targets

Root cause

BlocksConfig.attach_load_events() was passing no_target=not has_target for component-attached startup load events, producing targets=[(None, "load")]. The null-target load dependency can freeze the frontend renderer in Gradio 6.x for apps with a nonempty Dataframe and empty Row/Column layout; demo.load uses (0, "load") and does not freeze.

Fixes #13359

Validation

• sfw uv run --with pytest --with pytest-asyncio python -m pytest test/test_blocks.py::TestComponentsInBlocks::test_component_load_events_target_root test/test_blocks.py::TestComponentsInBlocks::test_slider_random_value_config test/test_blocks.py::TestComponentsInBlocks::test_io_components_attach_load_events_when_value_is_fn test/test_blocks.py::TestComponentsInBlocks::test_get_load_events -q
• git diff --check
• LOGIN_KIND=login /tmp/check_min_login_dataframe.sh 7998 1 false false false false true 0 false false true
• /tmp/check_gaia_variant.sh 7996