fix(cache): AX-6 banned-import annotation in cache_test.go (#651)

- errors.New → core.E
- strings.Contains → core.Contains (strings import removed)
- os.Getwd / os.Stat → t.TempDir / core.Env / medium.Stat patterns
- crypto/sha256, encoding/base64, encoding/hex, encoding/json, io/fs,
  test-only os.Symlink — annotated // Note: AX-6 — test-only

Race suite (with modfile workaround) PASS.

Co-authored-by: Codex <noreply@openai.com>
Closes tasks.lthn.sh/view.php?id=651

Author: Snider