perf(bigquery): avoid running list_tables when registering memtables

[cpcloud]

Closes #9216.

[cpcloud]

BigQuery looking good:

cloud in 🌐 falcon in …/ibis on  faster-bigquery-memtables is 📦 v9.1.0 via 🐍 v3.10.14 via ❄️   impure (ibis-3.10.14-env) took 24s
❯ pytest -m bigquery -n auto --dist loadgroup
====================================================================== test session starts ======================================================================
platform linux -- Python 3.10.14, pytest-8.2.2, pluggy-1.5.0
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=1778153405
rootdir: ...
configfile: pyproject.toml
plugins: benchmark-4.0.0, timeout-2.3.1, xdist-3.6.1, hypothesis-6.103.2, repeat-0.9.3, randomly-3.15.0, clarity-1.0.1, anyio-4.4.0, cov-5.0.0, pytest_httpserver-1.0.10, mock-3.14.0, snapshot-0.9.0
32 workers [1937 items]
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=================================================== 1570 passed, 30 skipped, 337 xfailed in 250.66s (0:04:10) ===================================================