Remove deprecated when() usage from dplyr methods

[Copilot]

Grouped summarize() operations could emit purrr’s deprecated when() warning.

• Removed the obsolete when() implementation from slice.SummarizedExperiment().
• Added regression coverage for grouped summaries:

pasilla |>
  group_by(condition) |>
  summarize(mean_count = mean(counts))

The repository now contains no when() calls in R source files.

• Fixes Deprecated 'when()' when running summarise() #123