fix(oracle): map ISO week-numbering year IYYY to %G [CLAUDE]

[vidigoat]

Oracle's TIME_MAPPING already maps IW (ISO 8601 week) to %V, but not IYYY (the ISO 8601 week-numbering year), so %G coming from another dialect is emitted into Oracle's TO_CHAR as a literal:

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

sqlglot.transpile("SELECT FORMAT_DATE('%G-%V', x)", read="bigquery", write="oracle")
# before: SELECT TO_CHAR(CAST(x AS DATE), '%G-IW')
# after:  SELECT TO_CHAR(CAST(x AS DATE), 'IYYY-IW')

Oracle documents IYYY as the "4-digit year of the year containing the calendar week, as defined by the ISO 8601 standard" (https://docs.oracle.com/en/database/oracle/oracle-database/23/sqlrf/Format-Models.html), which is what %G means. Per the same table, IW and IYYY are valid in TO_CHAR but not in the TO_* conversion functions, so the new tests only cover the TO_CHAR direction. This is the Oracle counterpart of #8672 (Postgres, merged).

Tests: two validate_all cases in tests/dialects/test_oracle.py (DuckDB and BigQuery into Oracle). Both fail on main and pass with this change; the full unittest suite (1477 tests) and pre-commit run --all-files (ruff, ruff-format, mypy) pass locally.

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