test(update): isolate integration test cache state

[ryanfowler]

Summary

• isolate update cache and lock state for update integration tests
• prevent parallel tests from suppressing auto-update checks through lock contention
• support platform-specific cache environment selection

Validation

• cargo fmt --check
• cargo test --locked --all-features --test update -- --test-threads=2
• cargo clippy --locked --all-targets --all-features -- -D warnings