Fix(clickhouse): don't generate parentheses, match R_PAREN conditionally

[georgesittas]

Addresses #2323 (comment) and fixes another generation bug.

References:

• https://clickhouse.com/docs/en/sql-reference/statements/select/sample
• https://play.clickhouse.com/play?user=play#U0VMRUNUIHN1bShmb28gKiBiYXIpIEZST00gYmxhIFNBTVBMRSAoMTAwMDAwMDAp

[georgesittas]

This turned out a bit awkward.. the motivation was to support stuff like SAMPLE 1 / 10 (shown as an example in ClickHouse's docs). The thing is that _parse_term doesn't suffice, because DuckDB also allows SAMPLE 10%, so the parser would try to parse this into a Mod and fail due to not finding a RHS. We also need to ensure we're calling _parse_factor against a number because otherwise it might consume the BUCKET token in SELECT a FROM test TABLESAMPLE (BUCKET 1 OUT OF 5 ON x).