fix: improve error message when expand/complete on grouping column (#1509)

[LeonidasZhak]

Problem

When calling expand() or complete() on a grouped data frame with a grouping column, the error message was confusing:

bob <- tibble(x = c(1, 2, 3), y = c("abe", "bob", "abe"))
bob_grouped <- bob |> group_by(x)
expand(bob_grouped, x, y)
#> Error in `reframe()`:
#> ℹ In argument: `expand(data = pick(everything()), ..., .name_repair = .name_repair)`.
#> ℹ In group 1: `x = 1`.
#> Caused by error:
#> ! object 'x' not found

The error "object 'x' not found" is misleading because x exists in the data, but it's a grouping column that isn't available in the data mask used by pick(everything()).

Solution

Added explicit checks in expand.grouped_df() and complete.grouped_df() to detect when the user tries to expand/complete on a grouping column and provide a clear, actionable error message:

expand(bob_grouped, x, y)
#> Error:
#> ! Can't expand on a grouping column.
#> i Column "x" is a grouping variable.
#> i Use `dplyr::ungroup()` first, or expand on non-grouping columns.

Changes

• R/expand.R: Added check for grouping columns in expand.grouped_df()
• R/complete.R: Added check for grouping columns in complete.grouped_df()
• tests/testthat/test-expand.R: Updated test to use snapshot testing
• tests/testthat/test-complete.R: Updated test to use snapshot testing
• tests/testthat/_snaps/expand.md: Added snapshot for new error message
• tests/testthat/_snaps/complete.md: Added snapshot for new error message

Validation

• All 1323 tests pass
• New error messages are clear and actionable
• No duplicate PRs found for issue Confusing error message when "expand" hits grouped data #1509

Related Issues

Closes #1509

[LeonidasZhak]

I am withdrawing this PR as part of a broader cleanup of an oversized automated contribution batch. I am sorry for the review noise and the pressure this may have put on maintainers. Going forward I will keep contributions to this project much more limited and higher-signal, ideally only one or two focused PRs at a time. Thank you for maintaining the project.