Use explicit namespace for stats::na.omit() in tests

[Copilot]

Addresses review feedback to use explicit namespace qualification for stats::na.omit() in test file for consistency with the rest of the codebase.

Changes

• Updated na.omit() to stats::na.omit() in tests/testthat/test-plot_mirror_distributions.R

This aligns with existing usage in R/utils-group.R and R/check_balance.R, improving code clarity and avoiding potential namespace conflicts.

# Before
y_labels <- built$layout$panel_params[[1]]$y$get_labels() |>
  na.omit() |>
  as.numeric()

# After
y_labels <- built$layout$panel_params[[1]]$y$get_labels() |>
  stats::na.omit() |>
  as.numeric()

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