test: add testutil.WaitBuffer and replace time.Sleep in tests (#22922)

WaitBuffer is a thread-safe io.Writer that supports blocking until
accumulated output matches a substring or custom predicate. It
replaces ad-hoc safeBuffer/syncWriter types and time.Sleep-based
poll loops in tests with signal-driven waits.

- WaitFor/WaitForNth/WaitForCond for blocking on output
- Replace custom buffer types in cli/sync_test.go and
  provisionersdk/agent_test.go
- Convert time.Sleep poll loops to require.Eventually/require.Never
  in cli/ssh_test.go, coderd/activitybump_test.go,
  coderd/workspaceagentsrpc_test.go, workspaceproxy_test.go, and
  scaletest tests

docs/about/contributing/backend.md

@@ -118,6 +118,7 @@ The Coder backend includes a rich suite of unit and end-to-end tests. A variety
   * [port.go](https://github.com/coder/coder/blob/main/testutil/port.go): select a free random port
   * [prometheus.go](https://github.com/coder/coder/blob/main/testutil/prometheus.go): validate Prometheus metrics with expected values
   * [pty.go](https://github.com/coder/coder/blob/main/testutil/pty.go): read output from a terminal until a condition is met
+  * [wait_buffer.go](https://github.com/coder/coder/blob/main/testutil/wait_buffer.go): thread-safe `io.Writer` that blocks until accumulated output contains a signal (`WaitFor`, `WaitForNth`, `WaitForCond`)
 
 ### [dbtestutil](https://github.com/coder/coder/tree/main/coderd/database/dbtestutil)