When the non-ephemeral batch endpoint returns 401 or 403 (expired
token, revoked credentials, JWKS rotation), _initialize_backend_batch
now switches is_current_batch_ephemeral to True and retries via the
ephemeral endpoint. This preserves traces that would otherwise be
lost due to the timing gap between client-side token validation and
server-side JWT decode.
The fallback only triggers on the non-ephemeral path to prevent
infinite recursion. If the ephemeral attempt also fails, trace_batch_id
is cleared normally.
Addresses the 2M+ failed push attempts in which valid client-side
tokens were rejected on the server.
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