Validate StarRocks macro set on a live cluster (syntax gaps the docs don't cover) #2398

Description/Context

Items the StarRocks docs are silent on — validate on the QA StarRocks cluster before declaring the macro set done:

1. cross join unnest(...) / cross join lateral json_each(...) — docs only show the comma-join lateral form; call sites use cross join {{ ... }}.
2. date_format / str_to_date pass-through of literal 'T'/'Z' in format strings (assumed standard MySQL behavior).
3. %f output width (6 digits assumed).
4. cast(varchar as date/datetime) NULL-on-failure behavior (documented for JSON/overflow, not exhaustively for varchar→date).
5. Confirm cluster version ≥ 3.3.5 (DATETIME µs floor).

Plan/Design

Build a small dbt integration test model exercising every starrocks__ variant with edge-case inputs (nulls, Z-suffix, offsets, malformed JSON, non-scalar JSON paths). Also verify last_value frame semantics with an explicit-frame test (SR ROWS default vs Trino RANGE).

Blocked by the implement-18-variants and fix-3-buggy-variants tasks. Related ready task: tk-verify-live-jwt-auth-test-in-qa-starrocks-cluste-de18c1 (cluster access).

P2. Part of #2375 (2026-07 dbt warehouse audit). Witan task: tk-validate-starrocks-macro-set-on-a-live-cluster-s-82d4c1.