Bigquery: Support DISTINCT / ORDER BY / LIMIT inside ARRAY_AGG and STRING_AGG

[hbridge]

Problem

The BigQuery grammar rejected all three GoogleSQL aggregate modifiers for ARRAY_AGG and STRING_AGG:

const { Parser } = require('node-sql-parser');
const p = new Parser();
const opt = { database: 'BigQuery' };

p.astify('SELECT ARRAY_AGG(DISTINCT x) AS a FROM t', opt);           // threw
p.astify('SELECT ARRAY_AGG(x LIMIT 5) AS a FROM t', opt);            // threw
p.astify('SELECT ARRAY_AGG(x ORDER BY y) AS a FROM t', opt);         // threw
p.astify('SELECT ARRAY_AGG(DISTINCT x ORDER BY x LIMIT 5) AS a FROM t', opt); // threw

These are all valid per the GoogleSQL ARRAY_AGG docs:

ARRAY_AGG([DISTINCT] expr [ORDER BY key [ASC|DESC] [, ...]] [LIMIT n])

Root cause

aggr_array_agg in pegjs/bigquery.pegjs matched the argument as a bare expr. Any modifier before the closing ) left unconsumed tokens and caused a parse error.

Fix

One rule change in pegjs/bigquery.pegjs: added an array_agg_arg rule that accepts the full modifier syntax, then wired aggr_array_agg to use it.

The new rule:

• Reuses existing building blocks — count_arg's KW_DISTINCT? / or_and_expr / order_by_clause? pattern is already tested and correct
• Mirrors the PostgreSQL distinct_args precedent already in pegjs/postgresql.pegjs
• Adds limit_clause? as the BigQuery-specific piece (LIMIT inside an aggregate is GoogleSQL-only)
• Returns { distinct, expr, orderby, limit } — same shape as count_arg, so aggrToSQL needs only one new line

src/aggregation.js gains one line: if (args.limit) str = [str, limitToSQL(args.limit)].join(' ').

Tests

Added to test/bigquery.spec.js:

• 6 round-trip tests (one per modifier combination for both ARRAY_AGG and STRING_AGG)
• 6 AST-shape tests asserting args.distinct, args.orderby, and args.limit are populated correctly
• Updated 1 pre-existing "struct expr" test expectation from ARRAY_AGG(undefined) (args were unreachable before) to the now-correctly-parsed output

Full suite: 1584 passing, 1 pending, 0 regressions.