test(layout): add comprehensive tests for CortexSidebarContainer

Create CortexSidebarContainer.test.tsx with 45 tests across 8 describe blocks:

- Panel Rendering (18 tests): verify correct panel renders for each of 9
  SidebarTab values (files, search, git, debug, extensions, agents, themes,
  plugins, account) and no other panels render simultaneously. Uses async
  findByTestId for lazy-loaded components with Suspense.
- Factory Panel Removal (2 tests): verify factory panel is not rendered for
  any valid sidebar tab, confirming removal from expected tab set.
- Sidebar Width (3 tests): verify sidebarWidth prop applies correctly to
  aside element style for various width values.
- Collapsed State (4 tests): verify aside, resize handle, and panel content
  are hidden when sidebarCollapsed is true.
- Resize Handle (6 tests): full mousedown/mousemove/mouseup interaction
  sequence including onResizingChange and onSidebarWidthChange callbacks,
  and event listener cleanup after mouseup.
- Width Constraints (5 tests): verify SIDEBAR_MIN_WIDTH (200) and
  SIDEBAR_MAX_WIDTH (600) clamping during resize drag.
- File Explorer Tab (5 tests): onFileSelect callback propagation, empty
  state for null/"." projectPath, and Open Folder button rendering.
- Transition Behavior (2 tests): transition disabled during resize,
  enabled otherwise.

All 10 lazy-loaded panel components are mocked via vi.mock(). All renders
use a Suspense wrapper as required by the lazy-loaded architecture.

Author: echobt