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/common.rs

@@ -3482,6 +3482,14 @@ mod tests {
                 codex_responses_as_items: None,
                 codex_response_item_prefix: None,
                 codex_response_handoff_mode: Some(CodexResponseHandoffMode::BemTags),
+                codex_response_handoff_channel_prefixes: Some(std::collections::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()]),
+                ])),
                 thread_id: "thr_123".to_string(),
                 model: Some("realtime-treatment-model".to_string()),
                 output_modality: RealtimeOutputModality::Audio,
@@ -3514,6 +3522,11 @@ mod tests {
                     "codexResponsesAsItems": null,
                     "codexResponseItemPrefix": null,
                     "codexResponseHandoffMode": "bemTags",
+                    "codexResponseHandoffChannelPrefixes": {
+                        "analysis": ["[THINKING]"],
+                        "commentary": ["[PROGRESS]", "[UPDATE]"],
+                        "final": ["[DONE]"]
+                    },
                     "model": "realtime-treatment-model",
                     "outputModality": "audio",
                     "includeStartupContext": false,
@@ -3549,6 +3562,7 @@ mod tests {
                 codex_responses_as_items: None,
                 codex_response_item_prefix: None,
                 codex_response_handoff_mode: None,
+                codex_response_handoff_channel_prefixes: None,
                 thread_id: "thr_123".to_string(),
                 model: None,
                 output_modality: RealtimeOutputModality::Audio,
@@ -3572,6 +3586,7 @@ mod tests {
                     "codexResponsesAsItems": null,
                     "codexResponseItemPrefix": null,
                     "codexResponseHandoffMode": null,
+                    "codexResponseHandoffChannelPrefixes": null,
                     "model": null,
                     "outputModality": "audio",
                     "includeStartupContext": null,
@@ -3593,6 +3608,7 @@ mod tests {
                 codex_responses_as_items: None,
                 codex_response_item_prefix: None,
                 codex_response_handoff_mode: None,
+                codex_response_handoff_channel_prefixes: None,
                 thread_id: "thr_123".to_string(),
                 model: None,
                 output_modality: RealtimeOutputModality::Audio,
@@ -3616,6 +3632,7 @@ mod tests {
                     "codexResponsesAsItems": null,
                     "codexResponseItemPrefix": null,
                     "codexResponseHandoffMode": null,
+                    "codexResponseHandoffChannelPrefixes": null,
                     "model": null,
                     "outputModality": "audio",
                     "includeStartupContext": null,
@@ -3837,6 +3854,7 @@ mod tests {
                 codex_responses_as_items: None,
                 codex_response_item_prefix: None,
                 codex_response_handoff_mode: None,
+                codex_response_handoff_channel_prefixes: None,
                 thread_id: "thr_123".to_string(),
                 model: None,
                 output_modality: RealtimeOutputModality::Audio,