Respect kShowNever when side panel opens

[orpheuslummis]

Summary

When "Show Sidebar" is set to Never in brave://settings/appearance, opening a side panel extension (or built-in panel like Bookmarks) no longer forces the sidebar control strip (icon bar) to appear. Only the panel content is shown.

Resolves brave/brave-browser#51271

The Problem

ShowSidebarAll() unconditionally showed both the control view (icon strip) and the side panel, ignoring kShowNever. This forced a two-column layout (icon strip + panel) even when the user explicitly disabled the sidebar, wasting horizontal space—especially painful on smaller displays or when using side panel extensions like Claude or Page Sidebar.

Changes

All changes are in sidebar_container_view.cc/h (+ browsertest):

Touch Point	What Changed
ShowSidebar()	New bool show_sidebar_control parameter (default true) controls whether the icon strip is shown alongside the panel
ShowSidebarAll()	Now checks kShowNever before passing show_control — hides icon strip when sidebar is disabled
CalculatePreferredSize()	Fixed early return that bailed when control was hidden, even if the panel was visible (returned {0,0} → panel got zero space)
Layout()	Zeroes control_view_width when control is not visible (follows existing IsFullscreenForCurrentEntry() pattern)
SetSidebarShowOption()	When switching to kShowNever while a panel is open, hides just the control view and keeps the panel

No changes to kShowAlways or kShowOnMouseOver behavior — all existing code paths are unaffected.

Verified Scenarios

Scenario	Result
kShowNever + extension opens panel	Panel only, no strip
kShowNever + built-in item opens panel	Panel only, no strip
kShowNever + panel closes	Everything hides
kShowAlways + panel opens (regression check)	Both visible (unchanged)
kShowOnMouseOver + panel opens (regression check)	Both visible (unchanged)
Switch Always→Never mid-panel	Strip hides, panel stays
Switch Never→Always mid-panel	Strip reappears
Animation (show/hide/interruption)	Correct in all cases
CalculatePreferredSize / Layout	Correct for all visibility combos

Known Limitation

BraveContentsViewUtil::GetRoundedCornersForContentsView checks IsSidebarVisible() (which reports control-view visibility). In kShowNever+panel mode, the content area's lower corner on the panel side uses the window-corner radius instead of the border radius. This is cosmetic only and suitable for a follow-up.

Test Plan

• Added ShowNeverHidesControlViewWhenPanelOpens browsertest
• Added SwitchToShowNeverWhilePanelOpen browsertest
• Manual: Set "Show Sidebar" to Never → open a side panel extension → verify only panel shows, no icon strip
• Manual: With panel open, toggle "Show Sidebar" between Always/Never → verify control strip appears/disappears correctly
• Manual: Close panel in Never mode → verify everything hides cleanly
• Verify existing SidebarContainerViewBrowserTest tests still pass