[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/protocol/src/protocol.rs

@@ -182,10 +182,16 @@ pub struct McpServerRefreshConfig {
 pub struct ConversationStartParams {
     /// Overrides the configured realtime architecture for this session only.
     pub architecture: Option<RealtimeConversationArchitecture>,
+    /// Whether Codex response handoffs are managed through explicit client append calls.
+    pub client_managed_handoffs: bool,
     /// Sends automatic Codex responses as realtime conversation items instead of handoff appends.
     pub codex_responses_as_items: bool,
     /// Optional prefix added to automatic Codex response items when `codex_responses_as_items` is set.
     pub codex_response_item_prefix: Option<String>,
+    /// Optional prefix added to automatic V1 Codex commentary sent with
+    /// `conversation.handoff.append` when `codex_responses_as_items` is not set. Final answers are
+    /// sent without the prefix.
+    pub codex_response_handoff_prefix: Option<String>,
     /// Overrides the configured realtime model for this session only.
     pub model: Option<String>,
     /// Selects whether the realtime session should produce text or audio output.