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

[nicoburns]

Objective

Fix ordinary (non-scroll-container) padded boxes propagating spuriously enlarged content sizes to ancestor scroll containers. In Blitz this manifested as a spurious ~34px horizontal scroll on https://csskit.rs at a 1200px viewport: the page's footer (a padded block whose child <p> has negative horizontal margins pulling it into the padding) reported content_size.width = 1234, which propagated up to the root and made the document horizontally scrollable.

Per CSS Overflow, a scroll container's own end-side padding is part of its scrollable overflow region, but a box that is not a scroll container does not extend its overflow region by its own padding. Browsers behave this way: content overflowing into (or past) a non-scroll-container's padding does not enlarge the scrollable overflow contributed to ancestors.

The block, flexbox and grid algorithms now only add the container's own end-side padding (right for LTR, left for RTL, plus bottom) to content_size when the container is a scroll container:

if is_scroll_container {
    content_size.width += if direction.is_rtl() { padding.left } else { padding.right };
    content_size.height += padding.bottom;
}

For flexbox this required threading an is_scroll_container flag through AlgoConstants (cfg-gated on content_size).

Context

• The unconditional padding addition was introduced to match browser scrollWidth/scrollHeight semantics for scroll containers; this PR keeps that behaviour for scroll containers (the existing content_size_includes_the_containers_own_padding / scroll_height_accounts_for_the_containers_own_padding tests still pass) and only removes it for non-scroll-containers.
• Added a content_size_excludes_the_own_padding_of_a_non_scroll_container test covering Block/Flex/Grid.
• Full test suite passes (5829 tests), plus a CHANGELOG entry under Unreleased → Fixed.
• Verified downstream in Blitz: with this fix the csskit.rs root goes from content_size=1234x3246, scroll_width=34 to content_size=1200x3246, scroll_width=0 (companion Blitz PR bumps the taffy rev).

Link to Devin session: https://dioxus.staging.devinenterprise.com/sessions/b6f5529e9a3e4823b603788b475ab109
Requested by: @nicoburns

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