fix(postgres)!: map ISO week format patterns IYYY and IW [CLAUDE]

[vidigoat]

Postgres' TIME_MAPPING has no entries for the ISO 8601 patterns IYYY (week-numbering year) and IW (ISO week), so the format trie splits IYYY into I + YYY and leaves IW untouched. The output is a format string that no target understands, and the reverse direction leaks %G/%V into TO_CHAR. This adds both patterns (upper and lower case, like the existing ww/WW) mapped to %G and %V, matching what Exasol already does.

sqlglot.transpile("SELECT TO_CHAR(x, 'IYYY-IW')", read="postgres", write="duckdb")
# before: SELECT STRFTIME(x, 'I%Ythree-IW')
# after:  SELECT STRFTIME(x, '%G-%V')

sqlglot.transpile("SELECT STRFTIME(x, '%G-%V')", read="duckdb", write="postgres")
# before: SELECT TO_CHAR(x, '%G-%V')
# after:  SELECT TO_CHAR(x, 'IYYY-IW')

I checked the results on PostgreSQL 17.11 and DuckDB 1.5.6: for 2021-01-01, 2024-12-30 and 2023-01-01, TO_CHAR(d, 'IYYY-IW') and STRFTIME(d, '%G-%V') both return 2020-53, 2025-01 and 2022-52. Before the fix, DuckDB returned I2021three-IW for the first date. Docs: https://www.postgresql.org/docs/current/functions-formatting.html (Redshift, which inherits this mapping, documents the same patterns: https://docs.aws.amazon.com/redshift/latest/dg/r_FORMAT_strings.html). The new validate_all case is in tests/dialects/test_postgres.py.

Developed with Claude Code (AI) under my direction; tests and make check pass locally.