sessions: fix sidebar layout when customizations toolbar changes (#306314)

* sessions: fix sidebar layout when customizations toolbar changes

- Listen to toolbar's onDidChangeMenuItems to trigger re-layout when
  items are added/removed (e.g., Plugins appearing after extension
  activation)
- Remove redundant height: 100% from sessions-list-control (flex
  sizing handles this)
- Reduce sessions section bottom margin from 12px to 6px
- Rename onDidToggleCollapse to onDidChangeLayout to reflect broader
  layout change handling

Co-authored-by: Copilot <[email protected]>

* Update src/vs/sessions/contrib/sessions/browser/media/sessionsList.css

Co-authored-by: Copilot <[email protected]>

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Co-authored-by: Copilot <[email protected]>
Co-authored-by: Copilot <[email protected]>