Make ThreadedPeriodicWorker fork friendly

[sl0thentr0py]

Summary

ThreadedPeriodicWorker is not currently fork friendly. After a process fork, only the thread that called fork survives in the child. The child receives copied Ruby objects and state, but the parent worker threads are not recreated.

A child can therefore inherit state such as:

@thread.alive? == false
@running == true
# or
@woken == true

No child thread may exist to clear the inherited state or broadcast the idle condition.

Concrete failure mode

TelemetryEventBuffer#flush calls wait_until_idle before anything calls ensure_thread. If the parent was running or had a pending wake when the fork happened, the child can wait indefinitely because its inherited worker thread is dead.

add_item commonly calls ensure_thread and may recreate a child worker when @exited is false, but that is not sufficient:

• wait_until_idle does not create a worker.
• wake does not create a worker.
• @exited may be inherited as true, preventing recreation.
• Inherited telemetry/session state can be sent by both the parent and child.

Proposed direction

Add fork-aware lifecycle handling for ThreadedPeriodicWorker and its subclasses:

• Detect a process change lazily using Process.pid (Ruby supports no public equivalent of Python's os.register_at_fork for the supported Ruby versions).
• Reset inherited worker state in the child before any wait or enqueue operation:
  • worker thread reference
  • @running
  • @woken
  • @exited
  • worker mutex and condition variables
• Recreate subclass synchronization state as needed.
• Discard inherited telemetry and session buffers in the child so parent-owned data is not sent twice.
• Allow the child to create a fresh worker lazily through ensure_thread.

The implementation should remain compatible with the current lifecycle decisions: keep Thread#kill, do not add cooperative shutdown or Thread.handle_interrupt, and do not rely on a global monkey-patch of Process.fork.

Acceptance criteria

• Forking while a worker is running does not make child wait_until_idle hang.
• Forking with a pending wake does not make child waits hang.
• Child telemetry/session buffers do not contain inherited parent data.
• The child can add new data and create a fresh worker.
• Parent worker behavior remains unchanged.
• Fork tests use bounded waits and are skipped where fork is unavailable.

Related context

The normal wake/run/wait protocol and post-exit behavior have already been hardened separately. This issue is specifically for process-fork state and inherited worker/buffer state.

[linear-code]

RUBY-213