Recycle brief if validation expands into multiple steps

[yjunechoe]

This PR aims to fix #478 by recycling brief when possible.

# Single string `brief` recycles
create_agent(small_table) |> 
  col_exists(c("a", "b"), brief = "one") |> 
  el("validation_set") |> 
  el("brief")
#> [1] "one" "one"

# Multi-length `brief` applies if exactly matched in number of steps produced
create_agent(small_table) |> 
  col_exists(c("a", "b"), brief = c("one", "two")) |> 
  el("validation_set") |> 
  el("brief")
#> [1] "one" "two"

# Errors if not matched in length (columns x segments produces 6 steps here)
create_agent(small_table) |> 
  col_vals_equal(
    c("a", "b"), 0,
    segments = vars(f),
    brief = c("one", "two")
  )
#> Error in `resolve_briefs()`:
#> ! `brief` must be length 1 or 6, not 2

This is a draft PR until I:

• Figure out the right level of abstraction for the resolve_brief() helper - namely, should it stand on its own (as current) or wrap over the default autobrief behavior, to host all brief-related logic? (will move forward with a unified resolve_brief() function)
• Do the necessary copy-pastes for all user-facing validation functions
• Write (batch?) tests
• Ensure yaml writing and round-tripping (similar to 75767a4)

Out of scope but worth considering for afterwards is whether we should support glue sytnax here as well.

[rich-iannone]

@yjunechoe just a note for future work on this PR: #579 changed settings in .Rproj to strip trailing whitespace. Merging from main and re-saving these files should fix the merge conflict.

[yjunechoe]

Got it - thanks for the heads up!

[yjunechoe]

@rich-iannone I think this is (finally) ready for review! The PR fixes the problem of brief failing to recycle across expanded steps. As a side effect, validation functions now generally support multi-length briefs, including a custom vector of briefs.

Internal details

The PR introduces resolve_brief() which handles the vector recycling as well as fallbacking to generate_autobriefs().

Batch tests are added to test-brief.R using the test_multi_briefs() helper, which will also print the agent report when ran interactively, for manual inspection of briefs.

TODO

I was thinking I should break up the PR as it's getting big. The one missing piece from this PR is supporting multi-length brief on the yaml read/write side of things. I plan to tackle this in a follow-up PR after this is merged.

I mentioned the possibility of supporting {glue} syntax in brief, like how we have it in label. If this is desirable, I think it should be simple to implement given the work in this PR, so this can be separately tackled in the near-term as well.

[rich-iannone]

LGTM!

[rich-iannone]

This is great, thanks for all the work you put into it! Will merge this now.