[codex] control automatic realtime handoff delivery (#27986)

## What

Built on the realtime speech-control plumbing merged in #27917.

- Add optional `codexResponseHandoffPrefix` to `thread/realtime/start`.
- Apply that prefix only to automatic V1 commentary sent through
`conversation.handoff.append`; final answers remain unprefixed.
- Add opt-in `clientManagedHandoffs`. When true, core suppresses
automatic response handoffs and completion output so delivery is
controlled by explicit client append APIs.
- Preserve existing automatic behavior by default.
`codexResponsesAsItems: true` continues to select item routing when
client-managed mode is disabled.

## Why

Voice clients need two delivery policies: automatic background context
with silent commentary instructions and fully client-owned handoffs.
Phase-aware prefixing keeps routine commentary silent without
suppressing the final answer, while client-managed mode lets an app
decide exactly which updates to append.

## Validation

- `just fmt`
- `cargo test -p codex-app-server-protocol
serialize_thread_realtime_start`
- `RUST_MIN_STACK=16777216 cargo test -p codex-core --test all
conversation_handoff_persists_across_item_done_until_turn_complete`
- `RUST_MIN_STACK=16777216 cargo test -p codex-app-server --test all
webrtc_v1_client_managed_handoffs_disable_automatic_output`
- `RUST_MIN_STACK=16777216 cargo test -p codex-app-server --test all
webrtc_v1_final_automatic_handoff_omits_silent_prefix`
- `cargo build -p codex-cli --bin codex`
- Local Codex Apps compatibility check: 43 focused webview tests passed,
and a live voice session routed through the source-built app-server.

The explicit `RUST_MIN_STACK` avoids a macOS Tokio test-worker stack
overflow seen with the default test environment.

codex-rs/app-server-protocol/src/protocol/common.rs

@@ -3082,8 +3082,10 @@ mod tests {
             request_id: RequestId::Integer(9),
             params: v2::ThreadRealtimeStartParams {
                 architecture: Some(RealtimeConversationArchitecture::Avas),
+                client_managed_handoffs: Some(true),
                 codex_responses_as_items: None,
                 codex_response_item_prefix: None,
+                codex_response_handoff_prefix: Some("silent context".to_string()),
                 thread_id: "thr_123".to_string(),
                 model: Some("realtime-treatment-model".to_string()),
                 output_modality: RealtimeOutputModality::Audio,
@@ -3102,8 +3104,10 @@ mod tests {
                 "params": {
                     "architecture": "avas",
                     "threadId": "thr_123",
+                    "clientManagedHandoffs": true,
                     "codexResponsesAsItems": null,
                     "codexResponseItemPrefix": null,
+                    "codexResponseHandoffPrefix": "silent context",
                     "model": "realtime-treatment-model",
                     "outputModality": "audio",
                     "includeStartupContext": false,
@@ -3125,8 +3129,10 @@ mod tests {
             request_id: RequestId::Integer(9),
             params: v2::ThreadRealtimeStartParams {
                 architecture: None,
+                client_managed_handoffs: None,
                 codex_responses_as_items: None,
                 codex_response_item_prefix: None,
+                codex_response_handoff_prefix: None,
                 thread_id: "thr_123".to_string(),
                 model: None,
                 output_modality: RealtimeOutputModality::Audio,
@@ -3145,8 +3151,10 @@ mod tests {
                 "params": {
                     "architecture": null,
                     "threadId": "thr_123",
+                    "clientManagedHandoffs": null,
                     "codexResponsesAsItems": null,
                     "codexResponseItemPrefix": null,
+                    "codexResponseHandoffPrefix": null,
                     "model": null,
                     "outputModality": "audio",
                     "includeStartupContext": null,
@@ -3163,8 +3171,10 @@ mod tests {
             request_id: RequestId::Integer(9),
             params: v2::ThreadRealtimeStartParams {
                 architecture: None,
+                client_managed_handoffs: None,
                 codex_responses_as_items: None,
                 codex_response_item_prefix: None,
+                codex_response_handoff_prefix: None,
                 thread_id: "thr_123".to_string(),
                 model: None,
                 output_modality: RealtimeOutputModality::Audio,
@@ -3183,8 +3193,10 @@ mod tests {
                 "params": {
                     "architecture": null,
                     "threadId": "thr_123",
+                    "clientManagedHandoffs": null,
                     "codexResponsesAsItems": null,
                     "codexResponseItemPrefix": null,
+                    "codexResponseHandoffPrefix": null,
                     "model": null,
                     "outputModality": "audio",
                     "includeStartupContext": null,
@@ -3367,8 +3379,10 @@ mod tests {
             request_id: RequestId::Integer(1),
             params: v2::ThreadRealtimeStartParams {
                 architecture: None,
+                client_managed_handoffs: None,
                 codex_responses_as_items: None,
                 codex_response_item_prefix: None,
+                codex_response_handoff_prefix: None,
                 thread_id: "thr_123".to_string(),
                 model: None,
                 output_modality: RealtimeOutputModality::Audio,