test(internal): add lifecycle stress / fuzzing harness for PeriodicThread (#18183)

## Description

Adds `tests/internal/test_periodic_stress.py` — a randomized lifecycle
fuzzer for `PeriodicThread` that interleaves
create / start / stop / join / awake / drop-last-ref / fork / gc /
thread-churn across a pool of `PeriodicThread` and `PeriodicService`
objects.

This is the harness that surfaced #17707 and #18040
(`PeriodicThread.awake()` blocking forever after `stop()`).

### What's in the file

- `test_periodic_thread_lifecycle_stress` — the randomized fuzzer.
  Default budget kept short for CI (`DD_STRESS_ITERS=200`, ~10s).
  Documented env vars for soaks (`DD_STRESS_ITERS`, `DD_STRESS_SECONDS`,
  `DD_STRESS_SEED`, `DD_STRESS_POOL`, `DD_STRESS_FORK_EVERY`,
  `DD_STRESS_TRACE_FILE`). Seed is printed on every run so a failing
  random seed can be pinned with `DD_STRESS_SEED=N`.
- `test_periodic_thread_concurrent_dealloc_race` — focused regression
  for #17485 (refcount TOCTOU between `std::thread` creation and the
  lambda acquiring the GIL).
- `test_periodic_thread_stop_without_join_then_fork_repeat` — focused
  regression for #16955 (`pthread_t` recycling after stop-without-join
  then fork).

### How to run it when modifying the periodic thread framework

The module docstring documents the exact commands. tl;dr:

```
# ~30s soak
DD_STRESS_ITERS=10000 scripts/run-tests -- -- tests/internal/test_periodic_stress.py

# 120s wall-clock soak (recommended under ASan/TSan)
DD_STRESS_SECONDS=120 scripts/run-tests -- -- tests/internal/test_periodic_stress.py

# Reproduce a failing seed (the seed is printed to stderr by every run)
DD_STRESS_SEED=12345 scripts/run-tests -- -- tests/internal/test_periodic_stress.py
```

## Testing

All three tests pass locally on Python 3.13 in the testrunner image:

```
tests/internal/test_periodic_stress.py::test_periodic_thread_lifecycle_stress PASSED
tests/internal/test_periodic_stress.py::test_periodic_thread_concurrent_dealloc_race PASSED
tests/internal/test_periodic_stress.py::test_periodic_thread_stop_without_join_then_fork_repeat PASSED
```

## Risks

None — test-only addition. Existing test discovery and CI suites are
unchanged.

`changelog/no-changelog` — test-only.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: brettlangdon <brett.langdon@datadoghq.com>
Co-authored-by: erwan.viollet <erwan.viollet@datadoghq.com>

Author: r1viollet