fix(utils): don't duplicate base placements in get_opposite_axis_placements

[kno-raziel]

Problem

get_opposite_axis_placements applies the flip-alignment duplication unconditionally:

if flip_alignment {
    let mut opposite_list = list.clone().into_iter().map(get_opposite_alignment_placement).collect();
    list.append(&mut opposite_list);
}

In the JS source (@floating-ui/utils) this block is nested inside if (alignment), so base placements (no alignment) are never duplicated:

if (alignment) {
  list = list.map((side) => `${side}-${alignment}`);
  if (flipAlignment) {
    list = list.concat(list.map(getOppositeAlignmentPlacement));
  }
}

Without the guard, a base placement comes back duplicated:

get_opposite_axis_placements(Placement::Top, true, Some(Alignment::Start), None)
// returns [Left, Right, Left, Right]   (expected: [Left, Right])

Fix

Guard the flip-alignment block with alignment.is_some() to match the JS behaviour (the list.map(...) step is already a no-op for base placements via get_placement(side, None), so only the duplication needed guarding).

Tests

Adds a #[cfg(test)] module porting the JS suite packages/utils/test/getOppositeAxisPlacements.test.ts (16 cases: side / start-alignment / end-alignment / rtl). packages/utils had no tests previously; these fail before the fix on the base-placement cases and pass after.

cargo test -p floating-ui-utils   # 16 passed

Found while building a Leptos UI library on top of this crate — thanks for the Rust port!

[DanielleHuisman]

Thank you for finding and reporting this issue. Please remove the unnecessary comments.