[party] Race fix (#37726)

Party contains a state field that contains ref count *and some other things*... so RefIfNonZero needs to pay attention to only consider the ref count portion.

Closes #37726

COPYBARA_INTEGRATE_REVIEW=https://github.com/grpc/grpc/pull/37726 from ctiller:flake-fightas-7 f2b3b3ff6d
PiperOrigin-RevId: 674514201

src/core/ext/transport/chaotic_good/server_transport.cc

@@ -240,7 +240,7 @@ auto ChaoticGoodServerTransport::DeserializeAndPushFragmentToNewCall(
       call_initiator->SpawnGuarded(
           "server-write", [this, stream_id = frame_header.stream_id,
                            call_initiator = *call_initiator,
-                           call_handler = std::move(call.handler)]() {
+                           call_handler = std::move(call.handler)]() mutable {
             call_destination_->StartCall(std::move(call_handler));
             return CallOutboundLoop(stream_id, call_initiator);
           });

src/core/lib/promise/party.cc

@@ -41,18 +41,18 @@ namespace grpc_core {
 // PartySyncUsingAtomics
 
 GRPC_MUST_USE_RESULT bool Party::RefIfNonZero() {
-  auto count = state_.load(std::memory_order_relaxed);
+  auto state = state_.load(std::memory_order_relaxed);
   do {
     // If zero, we are done (without an increment). If not, we must do a CAS
     // to maintain the contract: do not increment the counter if it is already
     // zero
-    if (count == 0) {
+    if ((state & kRefMask) == 0) {
       return false;
     }
-  } while (!state_.compare_exchange_weak(count, count + kOneRef,
+  } while (!state_.compare_exchange_weak(state, state + kOneRef,
                                          std::memory_order_acq_rel,
                                          std::memory_order_relaxed));
-  LogStateChange("RefIfNonZero", count, count + kOneRef);
+  LogStateChange("RefIfNonZero", state, state + kOneRef);
   return true;
 }