ping_pong: audit tests (#2007)

Summary:
Pull Request resolved: #2007

this completes the audit for ping_pong.rs. the single test here (test_sim_ping_pong) is V0-specific: it exercises the old multiprocess topology (`SystemActor` + `ProcActor` + `System::attach`) running over simulated networking. that model doesn't exist in V1, and while simnet itself is still available, there isn't a direct parity test to port — doing so would amount to adding new fault-injection coverage rather than closing a gap. the test is therefore annotated as V0-only.

with this file done, the V0→V1 test audit across hyperactor_multiprocess is complete. every V0 test is now categorized as V0-only, V1-equivalent, or needing V1 coverage (the only gap was stop-timeout/abort, which has already been added to V1). V0 tests remain in place until hyperactor_multiprocess is removed, but they are now explicitly marked so it’s clear which parts of the tree are legacy.

Reviewed By: dulinriley

Differential Revision: D87931337

fbshipit-source-id: 7ac7bff4a644955043a8252a22bef3169b201d31

Author: shayne-fletcher

hyperactor_multiprocess/src/ping_pong.rs

@@ -29,6 +29,24 @@ mod tests {
     use crate::proc_actor::spawn;
     use crate::system_actor::ProcLifecycleMode;
 
+    // V0-specific test - no V1 equivalent (but could be valuable).
+    // Tests V0's multiprocess infrastructure (SystemActor, ProcActor,
+    // System::attach) with simulated networking (simnet). Spawns 2
+    // ProcActors, spawns PingPongActors on them, verifies cross-proc
+    // message passing through simulated network channels.
+    //
+    // V1 port considerations:
+    // - Simnet is available in hyperactor crate (not V0-specific)
+    // - V1 port would test ProcMesh/ActorMesh with simulated channels
+    // - Could validate mesh coordination, fault tolerance, network
+    //   delays
+    // - Would be a *new* test capability, not equivalent coverage
+    // - V1 already has real-process integration tests (adequate for
+    //   basic correctness)
+    // - Main value would be deterministic fault injection testing
+    //   (network partitions, message loss, etc.)
+    // - Decision: Not porting for coverage parity. Could revisit for
+    //   advanced fault scenario testing if needed.
     #[tracing_test::traced_test]
     #[tokio::test]
     async fn test_sim_ping_pong() {