Generates a session ID to be used in all subsequent v2 and v3 calls. This must be called before authenticating the user.

The session-header value returned in the response headers must be captured and passed to POST /api/v2/user/loginUser.

Session reuse: Call this endpoint once and reuse the session token for the duration of your workflow. Do not call getApplicationDescription + loginUser before every API request — this causes unnecessary login overhead and has been observed to generate 1,600+ logins per sync run in production integrations, degrading server performance for all tenants on the instance.