fix: restore s->length > 0 guard in fetch-all detection

[Claude]

Reverts the removal of the s->length > 0 guard in OpRange's fetch-all access pattern classification. The guard is intentional behavior.

Changes

• Restored s->length > 0 && condition in fetch-all detection logic (src/server/stream_family.cc:1294)

Rationale

Empty streams should use the default access_kind classification (typically kRandom), not be classified as fetch-all. The fetch-all classification is specifically for operations that scan all existing entries in a non-empty stream.

Before (incorrect):

if (s->length == 0 || (streamCompareID(&sstart, &s->first_id) <= 0 && ...))

After (correct):

if (s->length > 0 && streamCompareID(&sstart, &s->first_id) <= 0 && ...)

[romange]

@claude do not remove s->length > 0 guard