feat(agent): wire up agentssh server to allow exec into container (#16638)

Builds on top of https://github.com/coder/coder/pull/16623/ and wires up
the ReconnectingPTY server. This does nothing to wire up the web
terminal yet but the added test demonstrates the functionality working.

Other changes:
* Refactors and moves the `SystemEnvInfo` interface to the
`agent/usershell` package to address follow-up from
https://github.com/coder/coder/pull/16623#discussion_r1967580249
* Marks `usershellinfo.Get` as deprecated. Consumers should use the
`EnvInfoer` interface instead.

---------

Co-authored-by: Mathias Fredriksson <mafredri@gmail.com>
Co-authored-by: Danny Kopping <danny@coder.com>

coderd/workspaceapps/proxy.go

@@ -653,6 +653,8 @@ func (s *Server) workspaceAgentPTY(rw http.ResponseWriter, r *http.Request) {
 	reconnect := parser.RequiredNotEmpty("reconnect").UUID(values, uuid.New(), "reconnect")
 	height := parser.UInt(values, 80, "height")
 	width := parser.UInt(values, 80, "width")
+	container := parser.String(values, "", "container")
+	containerUser := parser.String(values, "", "container_user")
 	if len(parser.Errors) > 0 {
 		httpapi.Write(ctx, rw, http.StatusBadRequest, codersdk.Response{
 			Message:     "Invalid query parameters.",
@@ -690,7 +692,10 @@ func (s *Server) workspaceAgentPTY(rw http.ResponseWriter, r *http.Request) {
 	}
 	defer release()
 	log.Debug(ctx, "dialed workspace agent")
-	ptNetConn, err := agentConn.ReconnectingPTY(ctx, reconnect, uint16(height), uint16(width), r.URL.Query().Get("command"))
+	ptNetConn, err := agentConn.ReconnectingPTY(ctx, reconnect, uint16(height), uint16(width), r.URL.Query().Get("command"), func(arp *workspacesdk.AgentReconnectingPTYInit) {
+		arp.Container = container
+		arp.ContainerUser = containerUser
+	})
 	if err != nil {
 		log.Debug(ctx, "dial reconnecting pty server in workspace agent", slog.Error(err))
 		_ = conn.Close(websocket.StatusInternalError, httpapi.WebsocketCloseSprintf("dial: %s", err))