Fix certificate generation

[awattez]

Description

Fix certificate generation reliability issues by:

1. Adding xargs to handle MacOS wc command whitespace output
2. Aligning return codes with shell conventions (0=success)
3. Renaming check_num_certs to check_truststore_valid for clarity

Prevents unnecessary certificate regeneration on MacOS while maintaining Linux compatibility.

Author Validation

• [x] Run cp-demo successfully on MacOS 14.7.1
• [x] Verified certificate validation works correctly
• [x] Confirmed no unnecessary regeneration occurs
• [x] Tested backward compatibility on Linux

Reviewer Tasks

• [ ] Run cp-demo to verify:
  • Certificate validation works
  • No unnecessary regeneration
  • Security features function correctly

[confluent-cla-assistant[bot]]

:tada: All Contributor License Agreements have been signed. Ready to merge.
_{Please push an empty commit if you would like to re-run the checks to verify CLA status for all contributors.}