Verifies that your current session is still active and returns the identity context it carries.

Use this endpoint to:

• Confirm a session is valid before making API calls.

• Inspect scope — see whether the session is a System session or
  is scoped to a specific customerUid.

• Diagnose auth failures — if subsequent API calls return 401,
  call this endpoint first to rule out an expired or revoked session.

Returns 200 with session details when the session is active. Returns 401 if the session is missing, expired, or the X-Client-Id does not match the session's credential.

This endpoint validates itself internally — no gateway fwdauth is required to call it.