test(examples): reap echo subprocess cleanly in test_echo_thin_waist (fixes #1541)

[yashksaini-coder]

Fixes #1541.

Problem

tests/examples/test_echo_thin_waist.py tears down the echo subprocess with terminate() then kill() — no proc.wait(), and it never closes proc.stdout. That leaks the stdout pipe (and, on Windows, the child echo server's listener socket, since SIGKILL gives it no chance to close gracefully).

Since #1533 enabled the global filterwarnings = ["error::ResourceWarning"] guard, that leak turns into a failure — windows (3.12, demos) failed while pytest processed the unraisable ResourceWarning (aggravated by a Windows pytest-tracemalloc hook bug). It is unrelated to the PR it surfaced on (#1532 / ICE-Lite), as noted in #1541.

Reproduced on Linux too: pytest tests/examples/test_echo_thin_waist.py -W all::ResourceWarning emits unclosed file <TextIOWrapper fd=13> (the never-closed proc.stdout).

Fix

Reap the child cleanly in the finally: terminate() → wait(timeout=5) → escalate to kill() + wait() only if it does not exit → then close() the stdout pipe. Also dropped the misleading "Trio primitives" note (the test is subprocess + time) and the unused monkeypatch/tmp_path fixtures — both flagged in #1541.

Verification

• pytest tests/examples/test_echo_thin_waist.py -W error::ResourceWarning → 1 passed (previously errored under the guard).
• -W all::ResourceWarning → 0 ResourceWarnings (was leaking the pipe).
• Swept tests/examples/ — this is the only subprocess.Popen test, so no other demos teardown needs the same fix. ruff/format clean.

[acul71]

Approval

Ready to merge. Clean, focused fix for #1541 that matches the maintainer teardown guidance.

Verified

• terminate() → wait(timeout=5) → kill() fallback → out_stream.close() correctly reaps the echo subprocess and closes the stdout pipe
• Unused monkeypatch/tmp_path removed; misleading "Trio primitives" note corrected
• Valid newsfragments/1541.internal.rst (ends with newline); Fixes #1541 linked
• Branch in sync with main (0 behind / 1 ahead), no merge conflicts
• CI fully green, including previously failing windows (3.12, demos)
• Local targeted check: passes under -W error::ResourceWarning with no ResourceWarnings

Non-blocking nits (optional follow-ups)

• Early proc.stdout is None path still terminate()s without wait()/close (practically unreachable with PIPE)
• Comment mentions SIGTERM/SIGKILL; on Windows these map to TerminateProcess — prefer terminate/kill wording

Merge readiness: Approve / squash-merge.