feat: add DROP pipe operator support

[jayendra13]

Which issue does this PR close?

• Part of Support SQL pipe operator #14660 (partial — adds DROP pipe operator only)

Rationale for this change

The DROP pipe operator (|> DROP x, y) is part of the BigQuery pipe syntax tracked in #14660. It removes columns from the output, equivalent to SELECT * EXCEPT(x, y).

What changes are included in this PR?

• Added PipeOperator::Drop match arm in query.rs that reuses existing SELECT * EXCEPT infrastructure (WildcardOptions with ExceptSelectItem)
• Added SLT tests covering single column drop, multi-column drop, chaining with other pipe operators, and error case for non-existent columns
• Added DROP to the pipe operator documentation in select.md

Are these changes tested?

Yes, four sqllogictest cases added in pipe_operator.slt:

• Drop two columns
• Drop single column
• Chained with WHERE
• Error on non-existent column

Are there any user-facing changes?

Yes, users can now use |> DROP column1, column2 in pipe operator queries (BigQuery dialect).

[simonvandel]

Very cool, thanks. The changes look good to me.

I'm not a committer, so someone else needs to do the final review.

[jayendra13]

Very cool, thanks. The changes look good to me.

I'm not a committer, so someone else needs to do the final review.

@simonvandel, whom can I ask for the final review ? @alamb, does this look good? If so, I can pick either RENAME or SET. Thanks.

[Jefffrey]

Thanks for picking this up; just some minor comments on tests, and once it's merged up from main it should be good to go

[Jefffrey]

Would be good to have a test case covering this

[jayendra13]

I think the https://github.com/google/googlesql/blob/master/docs/pipe-syntax.md#drop-pipe-operator makes column_name required (|> DROP column_name [, ...]), which would make this a parse error rather than a planner error, and it feels like it belongs in sqlparser instead. Could you confirm I'm reading the spec right? If so, I'll need to fix it in the upstream crate first.

[Jefffrey]

I'm not familiar with the spec, just basing it off the code I see here (if we need to ensure columns aren't empty here then it means its possible to be empty? Or its just safety?)

[Jefffrey]

Could we validate the specific error that comes here too?