Consolidate multi-agent v2 communication sends (#30867)

## Why

Multi-agent v2 communications currently use separate outbound paths:
direct messages, follow-up tasks, and completion results go through
`send_inter_agent_communication`, while a spawn's initial message goes
through the generic input submission path. That split makes it difficult
to add complete communication lifecycle logging in one place.

This refactor makes `submit_inter_agent_communication` the common sink
for those paths, preparing the follow-up observability work discussed in
[#30516](https://github.com/openai/codex/pull/30516).

## What changed

- Routed all current outbound `InterAgentCommunication` paths in
`AgentControl`—direct messages, follow-up tasks, completion results, and
multi-agent v2 spawn initial messages—through
`submit_inter_agent_communication`.
- Centralized the actual submission and last-task-message bookkeeping
there, providing one place for the follow-up PR to instrument
communication creation and successful enqueue.
- Left non-communication input handling and the multi-agent v1 spawn
flow unchanged.

## Testing

- `just test -p codex-core 'agent::control::tests::'` (51 passed)
- `just test -p codex-core
'suite::subagent_notifications::encrypted_multi_agent_v2_spawn_sends_agent_message_to_child'`
(passed)








---
[//]: # (BEGIN SAPLING FOOTER)
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with [ReviewStack](https://reviewstack.dev/openai/codex/pull/30867).
* #30872
* __->__ #30867

codex-rs/core/src/agent/control.rs

@@ -154,12 +154,13 @@ impl AgentControl {
         state: &Arc<ThreadManagerState>,
         initial_operation: Op,
     ) -> CodexResult<String> {
-        let last_task_message = match &initial_operation {
-            Op::InterAgentCommunication { communication } => {
-                last_task_message_from_communication(communication)
-            }
-            _ => non_empty_task_message(render_input_preview(&initial_operation)),
-        };
+        if let Op::InterAgentCommunication { communication } = initial_operation {
+            return self
+                .submit_inter_agent_communication(agent_id, state, communication)
+                .await;
+        }
+
+        let last_task_message = non_empty_task_message(render_input_preview(&initial_operation));
         let result = self
             .handle_thread_request_result(
                 agent_id,
@@ -182,13 +183,26 @@ impl AgentControl {
         &self,
         agent_id: ThreadId,
         communication: InterAgentCommunication,
+    ) -> CodexResult<String> {
+        self.send_input(agent_id, Op::InterAgentCommunication { communication })
+            .await
+    }
+
+    async fn submit_inter_agent_communication(
+        &self,
+        agent_id: ThreadId,
+        state: &Arc<ThreadManagerState>,
+        communication: InterAgentCommunication,
     ) -> CodexResult<String> {
         let last_task_message = last_task_message_from_communication(&communication);
-        let state = self.upgrade()?;
-        let op = Op::InterAgentCommunication { communication };
-        self.ensure_execution_capacity_for_op(agent_id, &op).await?;
         let result = self
-            .handle_thread_request_result(agent_id, &state, state.send_op(agent_id, op).await)
+            .handle_thread_request_result(
+                agent_id,
+                state,
+                state
+                    .send_op(agent_id, Op::InterAgentCommunication { communication })
+                    .await,
+            )
             .await;
         if result.is_ok() {
             match last_task_message {