Live share fix

[Fred-Wu]

File/content requests fixes to make live share work again

• Live Share callbacks now correctly unpack args arrays (host receives [context, path, encoding]).

Data viewer in guest given earlier data viewer model change

• Added Live Share path so guest data viewer can fetch rows through host’s R server.

Made guest-side more restrictive

• Guest only attaches to the host R session after explicit user action (Guest R: (not attached) → click to attach). No auto‑attach.
• Attach requests are ignored unless the host has a live R session (no server → no attach) and the guest actually requested attach.
• Guest UI/data access is gated by attachment:
  • Workspace view, data viewer, and plot viewer only update once attached.
  • Requests are dropped while not attached.
  • httpgd server could be re-attached via new command: R Live Share: Re-share plot server (httpgd)
• Guest is fully detached on host session end:
  • Workspace data cleared.
  • Data viewers and plot viewers closed.
  • Shared servers cleaned up.

Some other change

• dispose opened viewers when an extension restarts, but R session not ended but re-attachable. Otherwise, a new viewer would pop up if the old viewer is not closed during extension restart.