feat(duckdb): Add tests for TO_CHAR

[fivetran-amrutabhimsenayachit]

https://fivetran.atlassian.net/browse/RD-1069318

Problem: TO_CHAR(numeric, 'format') in Snowflake uses Oracle-style numeric format patterns (e.g. '99999.99'). When transpiling to DuckDB, the format argument was silently dropped, producing CAST(x AS TEXT) without any indication of the loss.

Solution: Confirmed the current CAST(x AS TEXT) + @unsupported_args("format") warning in the base generator is already the correct fallback — DuckDB has no native Oracle numeric format function and a partial translator would produce silently wrong results for several patterns (B, S, MI, EEEE). Added 4 integration tests to the submodule documenting the exact transpilation behavior for all cases: numeric (no format), TO_VARCHAR alias, date format (working via STRFTIME), and numeric with Oracle format (known limitation).

[github-actions]

SQLGlot Integration Test Results

Comparing:

• this branch (sqlglot:RD-1069318-to-char-to-varchar, sqlglot version: RD-1069318-to-char-to-varchar)
• baseline (sqlglot:cold-prql-test, sqlglot version: cold-prql-test)

By Dialect

dialect	sqlglot:cold-prql-test	sqlglot:RD-1069318-to-char-to-varchar	transitions	links
databricks -> databricks	1370/1370 passed (100.0%)	1370/1370 passed (100.0%)	No change	full result / delta
postgres -> postgres	6042/6042 passed (100.0%)	6042/6042 passed (100.0%)	No change	full result / delta
redshift -> redshift	7100/7101 passed (100.0%)	7101/7101 passed (100.0%)	1 fail -> pass	full result / delta
snowflake -> snowflake	63027/63027 passed (100.0%)	63027/63027 passed (100.0%)	No change	full result / delta
bigquery -> bigquery	23497/23497 passed (100.0%)	23497/23497 passed (100.0%)	No change	full result / delta

Overall

sqlglot:cold-prql-test: 101037 total, 101036 passed (pass rate: 100.0%), sqlglot version: cold-prql-test

sqlglot:RD-1069318-to-char-to-varchar: 101037 total, 101037 passed (pass rate: 100.0%), sqlglot version: RD-1069318-to-char-to-varchar

Transitions:
1 fail -> pass

✅ 62 test(s) passed