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.

support/support.go

@@ -390,7 +390,7 @@ func connectedAgentInfo(ctx context.Context, client *codersdk.Client, log slog.L
 		if err := conn.Close(); err != nil {
 			log.Error(ctx, "failed to close agent connection", slog.Error(err))
 		}
-		<-conn.Closed()
+		<-conn.TailnetConn().Closed()
 	}
 
 	eg.Go(func() error {
@@ -399,7 +399,7 @@ func connectedAgentInfo(ctx context.Context, client *codersdk.Client, log slog.L
 			return xerrors.Errorf("create request: %w", err)
 		}
 		rr := httptest.NewRecorder()
-		conn.MagicsockServeHTTPDebug(rr, req)
+		conn.TailnetConn().MagicsockServeHTTPDebug(rr, req)
 		a.ClientMagicsockHTML = rr.Body.Bytes()
 		return nil
 	})