feat(duckdb)!: Transpile BQ's WEEK based DATE_DIFF

[VaggelisD]

Follow up of #6354

BigQuery

BigQuery's DATE_DIFF has the following semantics:
1.WEEK is Sunday-based while ISOWEEK is Monday-based; One can also shift to any day through WEEK(<day>)
2. Date part boundaries are respected i.e crossings are counted for

This explains the following result:

# WEEK diff between Saturday and Sunday, counts as 1 because of WEEK (Sunday) crossing 
bq> SELECT DATE_DIFF('2026-01-04', '2026-01-03', WEEK);
1

# ISOWEEK diff counts as 0 because of ISOWEEK (Monday) is not crossed 
bq> SELECT DATE_DIFF('2026-01-04', '2026-01-03', WEEK);
0

DuckDB

In contrast, in DuckDB:

1. WEEK is Monday based
2. Crossings are not respected

# Monday crossing does NOT count: 
D SELECT DATE_DIFF('WEEK', '2026-01-03'::DATE, '2026-01-06'::DATE) AS num_weeks;
┌───────────┐
│ num_weeks │
│   int64   │
├───────────┤
│     0     │
└───────────┘

# Count increases only when day delta exceeds >= 7:
D SELECT DATE_DIFF('WEEK', '2026-01-03'::DATE, '2026-01-10'::DATE) AS num_weeks;
┌───────────┐
│ num_weeks │
│   int64   │
├───────────┤
│     1     │
└───────────┘

Solution

This PR attempts to match the semantics by:
a. [BigQuery] Canonicalizing WEEK to WEEK(Sunday) at parse time
b. [BigQuery] Adding a new arg in exp.DateDiff for "date part boundary" semantics
c. [DuckDB] Shifting the week start through DATE_TRUNC to account for (b)

Docs

BQ DATE_DIFF | DuckDB DATE_DIFF