#2703 tests: track the server sampler thread on shutdown

Descripción

# Issue: Sampler shutdown test inspects unrelated process threads

The test counts process-wide threads, so unrelated threads can make the server
shutdown assertion fail.

# PR: Check the server-owned sampler thread on shutdown

Assert the lifecycle of the sampler thread owned by the server under test.
No production code changes.

Validation: `serve.test_responses`, including the shutdown regression.

PR #1840 only waits for threads in its new `test_literal_think_guard.py`;
it does not change this existing assertion. A deterministic disappearing-thread
control fails on 296b59d2 and passes with this change.