fix(structs): ensure that isin works with struct membership

[cpcloud]

Fixes #8755.

[cpcloud]

Clouds are passing the new test:

…/ibis on  in-struct is 📦 v8.0.0 via 🐍 v3.12.2 via ❄️   impure (ibis-3.12.2-env) took 17s
❯ pytest 'ibis/backends/tests/test_struct.py::test_isin_struct' -m 'snowflake or bigquery'
======================================================================================= test session starts ========================================================================================
platform linux -- Python 3.12.2, pytest-8.1.1, pluggy-1.4.0
benchmark: 4.0.0 (defaults: timer=time.perf_counter disable_gc=False min_rounds=5 min_time=0.000005 max_time=1.0 calibration_precision=10 warmup=False warmup_iterations=100000)
Using --randomly-seed=1473849520
rootdir: /home/cloud/src/ibis
configfile: pyproject.toml
plugins: xdist-3.5.0, pytest_httpserver-1.0.10, cov-5.0.0, timeout-2.3.1, benchmark-4.0.0, randomly-3.15.0, hypothesis-6.100.1, repeat-0.9.3, snapshot-0.9.0, mock-3.14.0, clarity-1.0.1
collected 20 items / 18 deselected / 2 selected

ibis/backends/tests/test_struct.py ..                                                                                                                                                        [100%]

=========================================================================== 2 passed, 18 deselected in 83.82s (0:01:23) ============================================================================