Mode picker in Sessions app (#298199)

* feat: add mode picker functionality and integrate mode management in chat sessions

* feat: integrate IChatSessionsService and enhance mode picker with custom agent target

* feat: update mode picker to use MenuItemAction and remove setMode callback

* fix: update available modes logic to use effective target for custom agent selection

* feat: enhance mode picker to support configuration of custom agents and update item structure

* feat: integrate ILanguageModelToolsService and enhance mode handling in SessionsManagementService

* feat: add group property to configure custom agents action in mode picker

* feat: update session mode handling to reflect selected mode in input model

* fix setting agent in session options

* fix

* fix checking untitled session

* Update src/vs/sessions/contrib/chat/browser/modePicker.ts

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

* Update src/vs/sessions/contrib/chat/browser/newSession.ts

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

* Update src/vs/sessions/contrib/chat/browser/modePicker.ts

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

* Update src/vs/sessions/contrib/chat/browser/modePicker.ts

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

* feedback

* open new ai customisations editor

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