Fix new session focus from the chat sessions pane

[a77ming]

Fixes #291149.

Summary

• pass the active chat pane session context when the sessions pane New Session button runs the new chat action
• keep the action targeted at the chat pane instead of falling back to the last focused widget
• avoid focusing the welcome view input when it was the most recently focused chat widget

Testing

• Not run (not requested)

[a77ming]

Closing at author request.