Flaky test: tests/sentry/spans/consumers/process/test_consumer.py::test_schema_validator_rejects_none_fields[start_timestamp]

Failing test: `tests/sentry/spans/consumers/process/test_consumer.py::test_schema_validator_rejects_none_fields[start_timestamp]`
Sentry sha: 94d9d60db8e0e6f4937593247f1d6d178df1bcaa
Run: https://github.com/getsentry/sentry/actions/runs/22267329565

```
.venv/lib/python3.13/site-packages/_pytest/runner.py:340: in from_call
result: Optional[TResult] = func()
.venv/lib/python3.13/site-packages/_pytest/runner.py:240: in
lambda: runtest_hook(item=item, **kwds), when=when, reraise=reraise
.venv/lib/python3.13/site-packages/pluggy/_hooks.py:513: in __call__
return self._hookexec(self.name, self._hookimpls.copy(), kwargs, firstresult)
.venv/lib/python3.13/site-packages/pluggy/_manager.py:120: in _hookexec
return self._inner_hookexec(hook_name, methods, kwargs, firstresult)
.venv/lib/python3.13/site-packages/_pytest/threadexception.py:87: in pytest_runtest_call
yield from thread_exception_runtest_hook()
.venv/lib/python3.13/site-packages/_pytest/threadexception.py:63: in thread_exception_runtest_hook
yield
.venv/lib/python3.13/site-packages/_pytest/unraisableexception.py:90: in pytest_runtest_call
yield from unraisable_exception_runtest_hook()
.venv/lib/python3.13/site-packages/_pytest/unraisableexception.py:80: in unraisable_exception_runtest_hook
warnings.warn(pytest.PytestUnraisableExceptionWarning(msg))
E pytest.PytestUnraisableExceptionWarning: Exception ignored in: 
E 
E Traceback (most recent call last):
E File "/opt/hostedtoolcache/Python/3.13.1/x64/lib/python3.13/multiprocessing/resource_tracker.py", line 195, in _send
E self.ensure_running()
E ~~~~~~~~~~~~~~~~~~~^^
E File "/opt/hostedtoolcache/Python/3.13.1/x64/lib/python3.13/multiprocessing/resource_tracker.py", line 114, in ensure_running
E return self._reentrant_call_error()
E ~~~~~~~~~~~~~~~~~~~~~~~~~~^^
E File "/opt/hostedtoolcache/Python/3.13.1/x64/lib/python3.13/multiprocessing/resource_tracker.py", line 75, in _reentrant_call_error
E raise ReentrantCallError(
E "Reentrant call into the multiprocessing resource tracker")
E multiprocessing.resource_tracker.ReentrantCallError: Reentrant call into the multiprocessing resource tracker
E 
E During handling of the above exception, another exception occurred:
E 
E Traceback (most recent call last):
E File "/opt/hostedtoolcache/Python/3.13.1/x64/lib/python3.13/multiprocessing/util.py", line 216, in __call__
E res = self._callback(*self._args, **self._kwargs)
E File "/opt/hostedtoolcache/Python/3.13.1/x64/lib/python3.13/multiprocessing/synchronize.py", line 88, in _cleanup
E unregister(name, "semaphore")
E ~~~~~~~~~~^^^^^^^^^^^^^^^^^^^
E File "/opt/hostedtoolcache/Python/3.13.1/x64/lib/python3.13/multiprocessing/resource_tracker.py", line 191, in unregister
E self._send('UNREGISTER', name, rtype)
E ~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^
E File "/opt/hostedtoolcache/Python/3.13.1/x64/lib/python3.13/multiprocessing/resource_tracker.py", line 201, in _send
E warnings.warn(
E ~~~~~~~~~~~~~^
E f"ResourceTracker called reentrantly for resource cleanup, "
E ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
E f"which is unsupported. "
E ^^^^^^^^^^^^^^^^^^^^^^^^^
E f"The {rtype} object {name!r} might leak.")
E ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
E UserWarning: ResourceTracker called reentrantly for resource cleanup, which is unsupported. The semaphore object '/mp-yd6_ru1u' might leak.
```

Start with tests/sentry/spans/consumers/process/test_consumer.py::test_schema_validator_rejects_none_fields[start_timestamp] and reproduce the failure using the recorded Sentry SHA and Python 3.13 environment. Inspect the multiprocessing resource_tracker traceback and related test setup. Done means the test runs reliably without the reentrant cleanup warning or unraisable exception.

Written by the indexing model from the issue text.