fix: wait_agent timeout for queued mailbox mail (#18968)

## Why

`wait_agent` can be called while mailbox mail is already pending. The
previous implementation subscribed for future mailbox sequence changes
and then waited for the next notification. If the mail was queued before
that wait started, no new notification arrived, so the tool could sit
until `timeout_ms` even though mail was ready to deliver.

## What Changed

- Added `Session::has_pending_mailbox_items()` for checking pending
mailbox mail through the session API.
- Updated `multi_agents_v2::wait` to return immediately when pending
mailbox mail already exists before sleeping on a new mailbox sequence
update.
- Reworked the regression coverage in `multi_agents_tests.rs` so already
queued mailbox mail must wake `wait_agent` promptly.

Relevant code:
- [`wait_agent` pending-mail
check](https://github.com/openai/codex/blob/aa8ca06e83cf2a3dc22f86f37caec6cc2d9533ea/codex-rs/core/src/tools/handlers/multi_agents_v2/wait.rs#L55-L60)
-
[`Session::has_pending_mailbox_items`](https://github.com/openai/codex/blob/aa8ca06e83cf2a3dc22f86f37caec6cc2d9533ea/codex-rs/core/src/session/mod.rs#L2979-L2981)
-
[`multi_agent_v2_wait_agent_returns_for_already_queued_mail`](https://github.com/openai/codex/blob/aa8ca06e83cf2a3dc22f86f37caec6cc2d9533ea/codex-rs/core/src/tools/handlers/multi_agents_tests.rs#L2854)

## Verification

- `cargo test -p codex-core
multi_agent_v2_wait_agent_returns_for_already_queued_mail`

codex-rs/core/src/session/mod.rs

@@ -2976,6 +2976,10 @@ impl Session {
         self.mailbox_rx.lock().await.has_pending_trigger_turn()
     }
 
+    pub(crate) async fn has_pending_mailbox_items(&self) -> bool {
+        self.mailbox_rx.lock().await.has_pending()
+    }
+
     #[expect(
         clippy::await_holding_invalid_type,
         reason = "active turn checks and turn state updates must remain atomic"
@@ -3075,7 +3079,7 @@ impl Session {
         if !accepts_mailbox_delivery {
             return false;
         }
-        self.mailbox_rx.lock().await.has_pending()
+        self.has_pending_mailbox_items().await
     }
 
     pub async fn interrupt_task(self: &Arc<Self>) {