fix(e6): subtract 1 from EXTRACT(DOW) on the multidialect path

[tkaunlaky-e6]

Problem

e6's EXTRACT(DOW) is 1–7 (Sun=1); Postgres' is 0–6 (Sun=0). BI-tool (Tableau) queries emit 1 + EXTRACT(DOW) expecting Postgres' 0–6, so on e6 it becomes 1 + (1..7) = 2–8 (off by one).

Fix

Rewrite EXTRACT(DOW ..) → EXTRACT(DOW ..) - 1 so e6 reproduces the Postgres value in every context:

• 1 + EXTRACT(DOW) → 1 + (EXTRACT(DOW) - 1) = 1–7
• bare EXTRACT(DOW) → 0–6

subtract_one_from_dow lives in the e6 dialect (sqlglot/dialects/e6.py) and is applied via tree.transform(...) in the multidialect-only _region_to_e6 helper (converter_api.py). So it is gated on the MULTIDIALECT flag and covers both multidialect paths:

• primary (pg → databricks → e6)
• fallback (pg → e6)

Plain databricks → e6 (non-multidialect) is untouched. It's a plain AST rewrite, so no Databricks-generator change is needed — the primary path's intermediary is re-parsed by _region_to_e6 before e6 generation.

Verification (on the engine)

Schemaless (literal-date) queries transpiled through the converter and executed on e6:

Day	primary	fallback
Sunday	1	1
Monday	2	2
Saturday	7	7

Fallback genuinely exercised (numeric TRUNC forces the split; log confirms primary … failed → fallback). Non-multidialect control keeps 1 + EXTRACT(DOW) (no -1). e6 suite: 60 passed.