`vec_c()` and `list_unchop()` report bad error message for incompatible fallback objects in the >=3rd position

[DavisVaughan]

library(vctrs)

foobar <- function(x) {
  structure(x, class = "vctrs_foobar")
}

# Works
vec_c(
  foobar(1:2), foobar(5L)
)
#> [1] 1 2 5
#> attr(,"class")
#> [1] "vctrs_foobar"

# Correctly fails, good error
#
# This is a failure because `typeof` isn't the same between
# the two fallback objects
vec_c(
  foobar(1:2), foobar(0)
)
#> Error in `vec_c()`:
#> ! Can't combine `..1` <vctrs_foobar> and `..2` <vctrs_foobar>.
#> ✖ Some attributes are incompatible.
#> ℹ The author of the class should implement vctrs methods.
#> ℹ See <https://vctrs.r-lib.org/reference/faq-error-incompatible-attributes.html>.

# Correctly fails, bad error
#
# Common type of the first 2 is a `vctrs:::common_class_fallback`,
# and the common type of that and `foobar(5)` results in a failure,
# but the failure doesn't remove the `vctrs:::common_class_fallback`
# class in the failure report
vec_c(
  foobar(1:2), foobar(5L), foobar(0)
)
#> Error in `vec_c()`:
#> ! Can't combine `..1` <vctrs:::common_class_fallback> and `..3` <vctrs_foobar>.

# This also affects `list_unchop()`'s `default` argument, so we should
# add a test for that too
list_unchop(
  x = list(foobar(1:2), foobar(5:6)),
  indices = list(1:2, 5:6),
  size = 7,
  default = foobar(0)
)
#> Error in `list_unchop()`:
#> ! Can't combine <vctrs:::common_class_fallback> and `default` <vctrs_foobar>.

^{Created on 2025-07-15 with reprex v2.1.1}