Fix theming for sessions window (#297080)

* Refactor titlebar colors to use sidebar theme variables

* Add chat bar color variables and update titlebar styles

* Rename chat bar color variables to reflect header usage and update references in titlebarPart

* Rename chat bar header color variables to chat bar title colors and update references

* Update chat bar title color variables to use sidebar theme colors

* Add chat bar color variables and implement location-based color retrieval in ChatViewPane

* Refactor chat bar title colors to use editor background and foreground instead of sidebar colors

* Update chat bar title colors to use sidebar theme colors and remove unused color registrations

* Remove unused location-based color retrieval from ChatViewPane

* Refactor chat bar title colors to use sessions-specific header colors in SidebarPart and TitlebarPart

* Remove unused session sidebar header color registrations from chatColors.ts

* Add sessions sidebar background color and integrate into AgenticSessionsViewPane

* Add sessions sidebar border color and update SidebarPart to use it

* Update sessions sidebar border color to use contrastBorder for accessibility

* Update sessions sidebar border color to use specific hex values for better visibility

* Remove title bar border color from TitlebarPart to simplify styling