Propagate safety buffering events to app-server clients (#29371)

Responses API safety buffering metadata currently stops at the transport
boundary, so app-server clients cannot render the in-progress safety
review state.

This change:
- decodes and deduplicates `safety_buffering` metadata from Responses
API SSE and WebSocket events without suppressing the original response
event
- emits a typed core event containing the requested model plus backend
use cases and reasons
- forwards that event as `turn/safetyBuffering/updated` through
app-server v2 and updates generated protocol schemas
- keeps the side-channel event out of persisted rollouts and turn timing

This supports the Codex Apps buffering UX and depends on the Responses
API backend work in https://github.com/openai/openai/pull/1044569 and
https://github.com/openai/openai/pull/1044571.

Validation:
- focused `codex-core` safety-buffering integration test passes
- `cargo check -p codex-core -p codex-app-server -p
codex-app-server-protocol`
- `just fix -p codex-api -p codex-protocol -p codex-core -p
codex-app-server-protocol -p codex-app-server -p codex-rollout -p
codex-rollout-trace -p codex-otel`
- `just fmt`
- broad package test run: 4,430/4,492 passed; 62 unrelated
local-environment/concurrency failures involved unavailable test
binaries, MCP subprocess setup, and app-server timeouts

codex-rs/core/src/turn_timing.rs

@@ -339,6 +339,7 @@ fn response_event_records_turn_ttft(event: &ResponseEvent) -> bool {
         | ResponseEvent::ServerModel(_)
         | ResponseEvent::ModelVerifications(_)
         | ResponseEvent::TurnModerationMetadata(_)
+        | ResponseEvent::SafetyBuffering(_)
         | ResponseEvent::ServerReasoningIncluded(_)
         | ResponseEvent::ToolCallInputDelta { .. }
         | ResponseEvent::Completed { .. }