Only include a leaf's own end padding in content_size for scroll containers

[nicoburns]

Objective

Extend #1114 to leaf nodes: compute_leaf_layout previously set content_size = measured_size + padding.sum_axes(), unconditionally including both start and end padding. Block, flexbox and grid only include a container's own end-side padding in its content_size when it is a scroll container, so an ordinary padded leaf whose measured content overflows (e.g. clamped by max-size) over-reported its content size by the end padding.

compute_leaf_layout now computes:

content_size = Size {
    width: start_padding + measured_size.width + if is_scroll_container { end_padding } else { 0.0 },
    height: padding.top + measured_size.height + if is_scroll_container { padding.bottom } else { 0.0 },
}

where start/end padding follow the direction (left/right for LTR, right/left for RTL), matching the block/flexbox/grid convention exactly.

Context

Follow-up to #1114, which applied this convention to block/flexbox/grid but left compute_leaf_layout on the old unconditional behaviour. Adds leaf regression tests alongside the existing ones in tests/hand_written/scroll_size.rs and a changelog entry.

Link to Devin session: https://dioxus.staging.devinenterprise.com/sessions/cc8042ea8adf46a2a8d8e3431d560ae8
Requested by: @nicoburns

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