Resolvesbrave/brave-browser#52536
Refactors SidebarContainerView to support both Sidebar V1 and V2 behind the kSidebarV2 feature flag.
V1 - SidebarContainerView wraps and owns Chromium's SidePanel,
managing both the sidebar button bar and panel content.
V2 - SidebarContainerView manages only the sidebar button bar.
The side panel is left to upstream Chromium, removing the need to intercept
tab strip events or observe side panel entries.
Changes
SidebarContainerView - V2 paths added for construction, show/hide animation, layout, and preferred size.
V1-only methods guarded with CHECK(!IsV2Enabled()).
BraveBrowserView - V2 creates SidebarContainerView without a side panel.
V1 continues to re-parent Chromium's side panel into it.
BraveBrowserViewTabbedLayoutImpl - Null-checks side_panel() before setting margins, since V2 returns nullptr.
TEST=SidebarBrowserTest*
Simon Hong
ae30b7572f