[promise-based-endpoint] Instantiate an ExecCtx where required (#35505)

Closes #35505

COPYBARA_INTEGRATE_REVIEW=https://github.com/grpc/grpc/pull/35505 from ctiller:pbepec 54c023e589
PiperOrigin-RevId: 597845579

src/core/BUILD

@@ -6484,6 +6484,7 @@ grpc_cc_library(
         "slice",
         "slice_buffer",
         "//:event_engine_base_hdrs",
+        "//:exec_ctx",
         "//:gpr",
     ],
 )

src/core/lib/transport/promise_endpoint.cc

@@ -92,6 +92,8 @@ void PromiseEndpoint::ReadState::Complete(absl::Status status,
     }
     if (ep->Read(
             [self = Ref(), num_bytes_requested](absl::Status status) {
+              ApplicationCallbackExecCtx callback_exec_ctx;
+              ExecCtx exec_ctx;
               self->Complete(std::move(status), num_bytes_requested);
             },
             &pending_buffer, &read_args)) {

src/core/lib/transport/promise_endpoint.h

@@ -37,6 +37,7 @@
 #include <grpc/support/log.h>
 
 #include "src/core/lib/gprpp/sync.h"
+#include "src/core/lib/iomgr/exec_ctx.h"
 #include "src/core/lib/promise/activity.h"
 #include "src/core/lib/promise/if.h"
 #include "src/core/lib/promise/map.h"
@@ -82,6 +83,8 @@ class PromiseEndpoint {
       write_state_->waker = Activity::current()->MakeNonOwningWaker();
       completed = endpoint_->Write(
           [write_state = write_state_](absl::Status status) {
+            ApplicationCallbackExecCtx callback_exec_ctx;
+            ExecCtx exec_ctx;
             write_state->Complete(std::move(status));
           },
           &write_state_->buffer, nullptr /* uses default arguments */);
@@ -124,6 +127,8 @@ class PromiseEndpoint {
       read_state_->waker = Activity::current()->MakeNonOwningWaker();
       if (endpoint_->Read(
               [read_state = read_state_, num_bytes](absl::Status status) {
+                ApplicationCallbackExecCtx callback_exec_ctx;
+                ExecCtx exec_ctx;
                 read_state->Complete(std::move(status), num_bytes);
               },
               &read_state_->pending_buffer, &read_args)) {