[EventEngine] Enable the PosixEventEngine client experiment (#35985)

Closes #35985

COPYBARA_INTEGRATE_REVIEW=https://github.com/grpc/grpc/pull/35985 from drfloob:posix-ee-client-flip 15bab7a3b3
PiperOrigin-RevId: 684583835

bazel/experiments.bzl

@@ -135,16 +135,12 @@ EXPERIMENTS = {
         },
         "off": {
             "core_end2end_test": [
-                "event_engine_client",
                 "local_connector_secure",
             ],
             "endpoint_test": [
                 "tcp_frame_size_tuning",
                 "tcp_rcv_lowat",
             ],
-            "event_engine_client_test": [
-                "event_engine_client",
-            ],
             "flow_control_test": [
                 "multiping",
                 "tcp_frame_size_tuning",
@@ -160,6 +156,7 @@ EXPERIMENTS = {
                 "event_engine_dns",
             ],
             "core_end2end_test": [
+                "event_engine_client",
                 "event_engine_listener",
                 "work_serializer_dispatch",
             ],
@@ -169,6 +166,9 @@ EXPERIMENTS = {
             "cpp_lb_end2end_test": [
                 "pick_first_new",
             ],
+            "event_engine_client_test": [
+                "event_engine_client",
+            ],
             "event_engine_listener_test": [
                 "event_engine_listener",
             ],

src/core/lib/experiments/experiments.cc

@@ -397,7 +397,7 @@ const ExperimentMetadata g_experiment_metadata[] = {
      additional_constraints_event_engine_application_callbacks, nullptr, 0,
      true, true},
     {"event_engine_client", description_event_engine_client,
-     additional_constraints_event_engine_client, nullptr, 0, false, true},
+     additional_constraints_event_engine_client, nullptr, 0, true, true},
     {"event_engine_dns", description_event_engine_dns,
      additional_constraints_event_engine_dns, nullptr, 0, true, false},
     {"event_engine_listener", description_event_engine_listener,

src/core/lib/experiments/experiments.h

@@ -126,7 +126,8 @@ inline bool IsCanaryClientPrivacyEnabled() { return false; }
 inline bool IsClientPrivacyEnabled() { return false; }
 #define GRPC_EXPERIMENT_IS_INCLUDED_EVENT_ENGINE_APPLICATION_CALLBACKS
 inline bool IsEventEngineApplicationCallbacksEnabled() { return true; }
-inline bool IsEventEngineClientEnabled() { return false; }
+#define GRPC_EXPERIMENT_IS_INCLUDED_EVENT_ENGINE_CLIENT
+inline bool IsEventEngineClientEnabled() { return true; }
 #define GRPC_EXPERIMENT_IS_INCLUDED_EVENT_ENGINE_DNS
 inline bool IsEventEngineDnsEnabled() { return true; }
 #define GRPC_EXPERIMENT_IS_INCLUDED_EVENT_ENGINE_LISTENER

src/core/lib/experiments/rollouts.yaml

@@ -59,7 +59,7 @@
   default:
     # not tested on iOS at all
     ios: broken
-    posix: false
+    posix: true
     windows: true
 - name: event_engine_dns
   default: