feat(sess)!: stabilize attach identity and endpoint contract

[eitsupi]

Summary

On master, sess identity and discovery depend on local PIDs and ~/.vscode-R/sessions/<PID>.json. Reloading VS Code leaves surviving R processes disconnected, and restoring one terminal can prevent manual recovery of another. This PR establishes the session contract before the first stable 3.x release (#1763).

• Add a versioned attach handshake and process-lifetime session_id. Use socket ownership for cleanup so a late close cannot remove a replacement connection; defer workspace and plot notifications until task callbacks return, with generation guards across disconnects.
• Standardize sess::connect(endpoint = ...), SESS_ENDPOINT, and SESS_DISCOVERY_FILE. Store managed discovery files and attach scripts in extension storage, without making its directory layout part of the contract.
• Automatically reconnect each surviving managed terminal after reload, preserving identity and runtime options, restoring the selected terminal’s session, and refreshing the JGD renderer endpoint. Failed runtime startup closes the transport so reconnect can retry. Manual recovery prioritizes the selected terminal based on discovery ownership, not its name.
• Expose connection metadata and activation by session ID to downstream extensions. Share server startup so concurrent terminal and API clients receive the same endpoint. Clean up failed initialization and wait for startup before shutdown.
• Keep discovery extensible: version 1 requires only version and endpoint, ignores unknown fields, and permits optional backend fields such as jgdSocket without a version bump.

Breaking pre-release changes: replace pipe_path, SESS_PIPE, the pipe discovery field, and the PID-based home-directory fallback. Transport remains local Unix-domain sockets / Windows named pipes.

[Fred-Wu]

Given the recent large sess refactor, I may take the opportunity to ask whether you would consider supporting pseudo-terminal console applications. The only APIs that may be needed by downstream applications, such as my R Console, are:

1. Expose the connection details, such as the connection endpoint, protocol version, and plot backend address.
2. Allow activation of the connected session using its sess session ID.

In addition, during the review, there appears a bug. A VS Code reload may clean up the workspace callback while it is still sending notifications, which could crash R. It has been suggested to use later() around notify_client() so that the notification is sent after the workspace callback has finished.

[eitsupi]

@Fred-Wu Could you please take another look?
I believe I've prepared the necessary APIs for https://github.com/Fred-Wu/vscode-R-console.

[Fred-Wu]

Thanks for supporting the downstream applications. This might be another bug for native R terminals.

When VS Code is reloaded, R process survives, but sess connection is gone. I could restore the connection by clicking on R:(not attached). However if I have two R terminals opened, after reload, only one could be restored even both R processes survive. Click on R :(not attached) would switch to the other restored one.

Here is a small demo.

R.restore.mp4

[eitsupi]

@Fred-Wu I believe I've fixed the bug you pointed out, so could you please check it again?

[Fred-Wu]

Connections are now correctly restored automatically after VS Code reload.

But R Plot: commands are obsolete now. And they are not session aware at the moment.

Remove or fix in the next release?

[eitsupi]

But R Plot: commands are obsolete now. And they are not session aware at the moment.

Remove or fix in the next release?

Thanks for pointing that out!
I will take a look in #1762