Support configurable realtime BEM channel prefixes (#34816)

## What changed

- Route realtime V3 `bemTags` handoffs using `[ANALYSIS]`, `[COMMENTARY]`, and `[FINAL]` prefixes by default.
- Add `codexResponseHandoffChannelPrefixes` to realtime start requests so clients can replace the accepted prefixes for individual channels. Each channel may accept multiple prefixes, and omitted channels retain their defaults.
- Apply the configured prefixes to both complete and streamed handoff messages, without treating empty prefixes as matches.

## Testing

- Cover request deserialization, default and custom routing, partial streamed prefixes, empty-prefix handling, and end-to-end V3 handoffs.

GitOrigin-RevId: d0aef50aa71362a5beef2c6f6d28b8633da4e529

codex-rs/app-server-protocol/src/protocol/v2/tests.rs

@@ -4462,3 +4462,28 @@ fn realtime_start_omitted_initial_items_remain_none() {
 
     assert_eq!(params.initial_items, None);
 }
+#[test]
+fn realtime_start_deserializes_client_handoff_channel_prefixes() {
+    let params = serde_json::from_value::<ThreadRealtimeStartParams>(json!({
+        "threadId": "thread_123",
+        "outputModality": "audio",
+        "codexResponseHandoffChannelPrefixes": {
+            "analysis": ["[THINKING]"],
+            "commentary": ["[PROGRESS]", "[UPDATE]"],
+            "final": ["[DONE]"]
+        }
+    }))
+    .expect("params should deserialize");
+
+    assert_eq!(
+        params.codex_response_handoff_channel_prefixes,
+        Some(BTreeMap::from([
+            ("analysis".to_string(), vec!["[THINKING]".to_string()]),
+            (
+                "commentary".to_string(),
+                vec!["[PROGRESS]".to_string(), "[UPDATE]".to_string()],
+            ),
+            ("final".to_string(), vec!["[DONE]".to_string()]),
+        ]))
+    );
+}