bug: ApproxMedian with where clause applies where clause to the quantile argument

[ssabdb]

What happened?

t.float_col.approx_median(where=t.string_col > "a")

produces the SQL

SELECT
  APPROX_QUANTILES(
    IF(`t0`.`string_col` > 'a', `t0`.`float_col`, NULL),
    IF(`t0`.`string_col` > 'a', 2, NULL) <--- INCORRECT SQL HERE
  )[offset(1)] AS `ApproxMedian_float_col_Greater_string_col_'a'`
FROM `functional_alltypes` AS `t0`

which is incorrect, as the quantile spec must be a constant, e.g.

SELECT
  APPROX_QUANTILES(IF(`t0`.`string_col` > 'a', `t0`.`float_col`, NULL), 
  2 IGNORE NULLS)[1] <--- CORRECT SQL HERE
  AS `ApproxQuantile_float_col_0_5_Greater_string_col_'a'`
FROM `functional_alltypes` AS `t0`

What version of ibis are you using?

11.0.0

What backend(s) are you using, if any?

Bigquery

Relevant log output

Code of Conduct

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[ssabdb]

Integration tests suggests this is a problem in other backends as well, e.g. spark