Used columns in col_vals_expr() are extracted lazily

[yjunechoe]

In #505, the extraction of used columns was eager, at the validation step's creation. This forced evaluation of preconditions even when active was set to FALSE. To respect active and prevent materializing the table until if/when necessary, this PR simply moves the "extract used columns" behavior into interrogate().

Consequently, inactive steps revert to the old col_vals_expr() behavior of not showing used columns (as what columns are available in the table is is unknown until precondition is triggered)

(modified) reprex from #569

agent <- create_agent(tbl = iris) |> 
  col_vals_expr(~ Petal.Volume > Petal.Width,
                preconditions = \(x) x[x$Petal.Volume > -1, ],
                active = has_columns(iris, Petal.Volume)
  ) |>
  interrogate()

str(
  agent$validation_set[, c("eval_active", "column")]
)
#> tibble [1 × 2] (S3: tbl_df/tbl/data.frame)
#>  $ eval_active: logi FALSE
#>  $ column     :List of 1
#>   ..$ : chr NA

For the same reason, uninterrogated agents will not show used columns for col_vals_expr(). But interrogated agents will.

agent_uninterrogated <- create_agent(tbl = iris) |> 
  col_vals_expr(~ Petal.Length > Petal.Width)
str(
  agent_uninterrogated$validation_set[, c("eval_active", "column")]
)
#> tibble [1 × 2] (S3: tbl_df/tbl/data.frame)
#>  $ eval_active: logi NA
#>  $ column     :List of 1
#>   ..$ : chr NA

agent_interrogated <- agent_uninterrogated %>% 
  interrogate()
str(
  agent_interrogated$validation_set[, c("eval_active", "column")]
)
#> tibble [1 × 2] (S3: tbl_df/tbl/data.frame)
#>  $ eval_active: logi TRUE
#>  $ column     :List of 1
#>   ..$ : chr [1:2] "Petal.Length" "Petal.Width"

Active and interrogated col_vals_expr() steps will continue to show used columns in the report:

get_agent_report(agent_interrogated)

[rich-iannone]

This is great, thank you!

[rich-iannone]

(Feel free to merge whenever.)