Tests should not "spam" system directories

### Describe the bug

Some tests write test data to the system global `/tmp`:
```
/tmp/buckets
/tmp/ns
/tmp/ns1
/tmp/PolarisOverlappingTableTest
/tmp/test-private*.pem
/tmp/test-public*.pem
```

This is bad practice and should be replaced with JUnit's `@TempDir`. It can also easily cause issues when tests are run repeatedly.

Search the test sources for the listed global paths, including /tmp/buckets, /tmp/ns, /tmp/PolarisOverlappingTableTest, and the test PEM files, and inspect existing JUnit @TempDir usage. Replace those system-directory writes with temporary test directories, then run the affected tests repeatedly to confirm they pass without leaving global files.

Written by the indexing model from the issue text.