fix: fall back to Redfish session token auth in baremetal agent

[mvanhorn]

Summary

The baremetal agent's Redfish client now falls back to Session-token (X-Auth-Token) authentication when Basic Auth is rejected, so Redfish endpoints that require session auth work instead of failing the probe.

Why this matters

Some Redfish/BMC implementations reject Basic Auth and require a session token (#24911). The client only tried Basic Auth, so those endpoints failed. This reuses the existing Login/SessionToken machinery in pkg/util/redfish/redfish.go: when the initial probe returns an authentication error (401/403), it logs in for a session token and retries. Non-auth failures (404/500) fast-fail as before and never create an unnecessary session, detected via errors.As on the wrapped JSONClientError rather than a type assertion through errors.Cause.

This covers the authentication half of the issue; the non-standard System ID navigation path is already handled by the existing index-based Members traversal.

Testing

• Added tests: a 401 probe falls back to Login and retries; 404 and 500 do not fall back and create no session.
• go build and go test pass for pkg/util/redfish.

Refs #24911