SentryDriver.resolveOrgSlug duplicated the same /organizations/?member=true
call already implemented in ListSentryOrganizations, which is consumed by
the OAuth org picker. Delegating to the shared helper prevents the two
call sites from drifting (response shape, header set, pagination) and
keeps the driver focused on member listing.
Pure refactor: no behavior change. Add an httptest-backed smoke test
covering the empty-stored-slug path end-to-end through ListAccounts so
the auto-discovery flow stays exercised after the refactor.
Signed-off-by: Aurélien Sibiril <81782+aureliensibiril@users.noreply.github.com>