feat: Animate side navigation collapse and expandable section reveal

[jkuelz]

Description

Animates SideNavigation's collapsed icon rail transition and ExpandableSection's content reveal, scoped to 1T only as per the animation strategy and to prevent breaking changes.

• Replaces ExpandableSection's display: none/block content toggle with an animated grid-template-rows reveal. The content stays mounted and inert while collapsed, to allow for smooth transition states that weren't possible with the previous display: none.
• Refactors SideNavigation's to the same grid-template-rows strategy as expandable sections to enable transitions: text fade, spatial collapse/expand, and staggered enter/exit timing for links, sections, section-groups, link-groups, and expandable-link-groups.
• Adds a one-theme-with-motion mixin (internal/styles/motion/mixins.scss) alongside the existing with-motion, so animation-only CSS additions can be scoped to one-theme without affecting other themes' timing.
• Gives one-theme its own values, reusing the existing token pair rather than introducing new ones. These may change with the upcoming animations strategy.

Other behavior changes:

• Added warnings when a collapsed-rail item (a root-level link, link-group, or expandable-link-group, or a link/link-group/ELG nested inside a root-level section/section-group) has no icon and would therefore be invisible or have no visual indicator in the collapsed rail.
• Expandable-link-group children are now forced closed when the nav is collapsed, regardless of the ELG's own expanded/collapsed state or icon presence. Previously an ELG's would either collapse their children or show their children depending on whether the parent had an icon.
• Sections are now forced open when the nav is collapsed, so their icon-bearing children always flatten into the rail — this existed conceptually before but is now explicit and paired with the ELG behavior above.
• Sections, link-groups, and ELGs use inert (not just visual hiding) to remove their headers/parent links from the tab order and assistive technology when they collapse to zero size in the rail.
• Reverted an earlier change that added dividers between sections/groups in the collapsed state and returned to using space instead.
• New ariaLabel plumbing and aria-hidden on section/section-group header text for collapsed-rail accessibility.

Why animations only in 1T?
The initial version of this animation applied to all themes and broke the pre-existing contract for ExpandableSection that collapsed/hidden content is visually removed instantly (display: none) — which means any downstream tests that asserted isDisplayed()/isExisting() immediately after a collapse toggle with no wait, could fail. Scoping the animation to one-theme (opt-in) avoids that regression for existing VR/classic consumers while still shipping the new motion for one-theme.

Related links, issue #, if available: n/a

How has this been tested?

dev pipeline

Review checklist

The following items are to be evaluated by the author(s) and the reviewer(s).

Correctness

• Changes include appropriate documentation updates.
• Changes are backward-compatible if not indicated, see CONTRIBUTING.md.
• Changes do not include unsupported browser features, see CONTRIBUTING.md.
• Changes were manually tested for accessibility, see accessibility guidelines.

Security

• If the code handles URLs: all URLs are validated through the checkSafeUrl function.

Testing

• Changes are covered with new/existing unit tests?
• Changes are covered with new/existing integration tests?

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