Data Connections + Assistant: Expose UX in Assistant to attach data connections

## Context As a user who wants to ask questions about their active connections in Assistant: 1. Within Assistant, they may not know of the New Data Connections Pane where they can connect their data 2. They may want to chat only about 1 or several of their connected data connections 3. They may want to chat about a specific table within one of their data connections. 4. They may want to chat about the same data connection often Note, we started setting up data connection context here: https://github.com/posit-dev/positron/issues/14658 ## AC 1. Make it easy to connect to data from Assistant, for example offer a button within the Assistant UI "Connect Data" which opens up the new data connections pane <img width="1440" height="840" alt="Image" src="https://github.com/user-attachments/assets/db80712f-1ee3-40af-9a5a-36a19bbfd584" /> 2. Update Attach to allow you to select active data connections in addition to files and attach a specific connection as context to the chat. 3. Allow a user to select a specific table from a connection to add as context <img width="1440" height="344" alt="Image" src="https://github.com/user-attachments/assets/8572cd2e-541e-4c1a-ab7f-90e0dc872933" /> 4. The chat should show the attached connection context 5. A user should be able to attach multiple tables/ connections as context ## Design Examples <img width="1442" height="913" alt="Image" src="https://github.com/user-attachments/assets/32579ebf-857c-438f-8924-4cc1f4829abf" /> <img width="1435" height="904" alt="Image" src="https://github.com/user-attachments/assets/e5f55d48-206a-4887-a249-aa1c06ae0660" />