fix integration test transmits z/OS credentials over cleartext HTTP

[js665999]

• Default scheme changed from hardcoded http:// to https://, configurable via a new ZSS_PROTOCOL env var if someone genuinely needs cleartext for local-only testing (not recommended, but not silently forced away from them either).
• Added an httpsAgent to the shared axios instance so it works against a typical z/OS test ZSS with a self-signed certificate — but rejectUnauthorized defaults to true (secure), and only relaxes if the developer explicitly sets ZSS_ALLOW_SELF_SIGNED=true. Commented inline that it should never be used against a non-test endpoint.
• Since every request in the suite goes through the single instance (confirmed via grep — no bare axios.get() calls bypassing it), this covers all endpoints, including the ones that override baseURL per-call (gettcpipname, the malformed-tcpip-name cases, etc.).

[1000TurquoisePogs]

Looks good to me so far.