refactor: convert workspacesdk.AgentConn to an interface (#19392)

Fixes https://github.com/coder/internal/issues/907

We convert `workspacesdk.AgentConn` to an interface and generate a mock
for it. This allows writing `coderd` tests that rely on the agent's HTTP
api to not have to set up an entire tailnet networking stack.

cli/ping.go

@@ -147,7 +147,7 @@ func (r *RootCmd) ping() *serpent.Command {
 			}
 			defer conn.Close()
 
-			derpMap := conn.DERPMap()
+			derpMap := conn.TailnetConn().DERPMap()
 
 			diagCtx, diagCancel := context.WithTimeout(inv.Context(), 30*time.Second)
 			defer diagCancel()
@@ -156,7 +156,7 @@ func (r *RootCmd) ping() *serpent.Command {
 			// Silent ping to determine whether we should show diags
 			_, didP2p, _, _ := conn.Ping(ctx)
 
-			ni := conn.GetNetInfo()
+			ni := conn.TailnetConn().GetNetInfo()
 			connDiags := cliui.ConnDiags{
 				DisableDirect:      r.disableDirect,
 				LocalNetInfo:       ni,