perf: optimize get_interface_attr collection overhead

[brianescutia]

Summary

Optimizes get_interface_attr() in opendbc/car/interfaces.py to reduce pytest collection overhead.

Changes:

• iterate over canonical BRANDS instead of walking BASEDIR
• cache brand directory discovery
• cache interface attribute lookups
• avoid failed dynamic imports from non-brand directories

Motivation

While investigating #1184, I profiled opendbc/car/tests/ collection and found time spent in get_interface_attr() from filesystem walking and dynamic imports.

This change keeps the same public return shape while returning a fresh dict from cached tuple data to avoid cache poisoning if callers mutate the result.

Benchmarks

Before:

uv run pytest --cache-clear --co opendbc/car/tests/
# 763 tests collected in ~1.13s

After:

uv run pytest --cache-clear --co opendbc/car/tests/
# 763 tests collected in ~0.74s

Validation

uv run pytest opendbc/car/tests/test_car_interfaces.py -x
# 246 passed

uv run pytest opendbc/car/tests/test_fw_fingerprint.py -x
# 14 passed

uv run pytest opendbc/car/tests/test_docs.py -x
# 6 passed

uv run pytest opendbc/car/tests/test_platform_configs.py -x
# 1 passed

./test.sh
# All good! Finished lint and test

[github-actions]

Car behavior report

Replays driving segments through this PR and compares the behavior to master.
Please review any changes carefully to ensure they are expected.

✅ No changes detected

[github-actions]

Thanks for contributing to opendbc! In order for us to review your PR as quickly as possible, check the following:

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[brianescutia]

tests / test models (1) failed again due to an unavailable Rivian route:

c70d59e4150956fc/0000006e--48bfbfda01

This appears unrelated to this PR since ./test.sh, car diff, safety checks, and the other model shards passed, and the car behavior report shows no changes detected.