sessions: fix multi-select context menu to operate on all selected items (#306332)

* sessions: fix multi-select context menu to operate on all selected items

Context menu actions in the sessions list previously only operated on the
right-clicked item, ignoring other selected items. Now the context menu
collects the tree selection and passes all selected sessions to actions.

Changes:
- sessionsList.ts: Pass selected sessions array as arg to context menu actions
- sessionsViewActions.ts: Update all 7 item actions to accept ISession | ISession[]
- copilotChatSessionsActions.ts: Update bridge commands to handle session arrays

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

* ensure right-clicked element is first in multi-selection

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

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