[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/src/request_processors/turn_processor.rs

@@ -931,8 +931,10 @@ impl TurnRequestProcessor {
             thread.as_ref(),
             Op::RealtimeConversationStart(ConversationStartParams {
                 architecture: params.architecture,
+                client_managed_handoffs: params.client_managed_handoffs.unwrap_or(false),
                 codex_responses_as_items: params.codex_responses_as_items.unwrap_or(false),
                 codex_response_item_prefix: params.codex_response_item_prefix,
+                codex_response_handoff_prefix: params.codex_response_handoff_prefix,
                 model: params.model,
                 output_modality: params.output_modality,
                 include_startup_context: params.include_startup_context.unwrap_or(true),