fix(dbt): fix 3 buggy starrocks__ cross-db macro variants

[blarghmatey]

What are the relevant tickets?

Closes #2397

Description (What does it do?)

Fixes 3 correctness bugs in the starrocks__ dispatch variants of macros/cross_db_functions.sql, doc-verified against docs.starrocks.io as part of the dbt warehouse semantic-accuracy audit's StarRocks cross-database macro epic:

1. starrocks__json_extract_value used json_extract(), which does not exist in StarRocks. Switched to json_query(parse_json(x), path).
2. starrocks__from_iso8601_timestamp did a bare cast(x as datetime). StarRocks DATETIME is zone-less, so any string with a trailing Z or ±HH:MM offset made the cast silently return NULL. Now strips a trailing zone suffix and normalizes the T separator to a space before casting. Non-UTC offsets are dropped rather than applied — acceptable since this project's data is UTC/naive.
3. starrocks__array_join silently dropped the null_replacement argument even though StarRocks' array_join supports the same 3-arg null_replace_str signature as Trino (per StarRocks docs: array_join([1,3,5,null],'_','NULL')). Mirrored the default__ (Trino) branch structure.

How can this be tested?

• dbt parse against the dev_local DuckDB target succeeds (verified) — these macros aren't dispatched today since no model targets StarRocks yet, so this is Jinja-syntax + doc-verification, not a live StarRocks run.
• Once a StarRocks target/profile exists, exercise each macro directly, e.g.:
  • select {{ json_extract_value("'{\"a\":1}'", "'$.a'") }}
  • select {{ from_iso8601_timestamp("'2024-01-01T00:00:00Z'") }}
  • select {{ array_join('array(1,3,5,null)', '_', 'NULL') }}
    against a live StarRocks warehouse and confirm non-NULL, correctly-formatted results.

Additional Context

Part of tk-epic-starrocks-support-for-cross-database-dbt-ma-968247 in the dbt warehouse audit — a sibling task (tk-implement-18-missing-starrocks-macro-variants-do-57059b) tracks implementing the 18 currently-missing starrocks__ macro variants; not addressed here.

🤖 Generated with Claude Code

[Copilot]

Pull request overview

This PR updates the StarRocks-dispatched variants of existing cross-database dbt macros in src/ol_dbt/macros/cross_db_functions.sql to align with StarRocks function availability and parsing behavior, improving semantic correctness for a future StarRocks target.

Changes:

• Fix starrocks__json_extract_value to use json_query(parse_json(...), path) instead of a non-existent json_extract.
• Fix starrocks__from_iso8601_timestamp to strip timezone suffixes and normalize the T separator before casting to StarRocks DATETIME.
• Fix starrocks__array_join to correctly pass the optional null_replacement (3rd argument) when provided.

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