auth stabilisation: extract shared myportal auth guard helper
ntroduces a small shared SSR helper (resolveMyPortalAuthContext) to standardize common myportal auth/session guards (session presence, session user identity, and pinsUser cookie) with preserved reason-coded diagnostics and signin redirect behavior. Migrates exactly two loaders (pages/myportal/searchresults.js, pages/myportal/addresssearchresults.js) to use the helper while keeping loader-specific UPSTREAM_FAILURE and CONTACT_LOOKUP_FAILED logic unchanged.
Related work items: #23020
Robert Bond