Always assign response item IDs (#34645)

## What changed

- Assign IDs to client-created response items in every session, including streamed items, forked history, compaction results, and non-OpenAI provider requests.
- Preserve server-provided IDs and stable generated IDs across persisted and resumed conversations while continuing to omit unprefixed server IDs from outbound requests.
- Retire `features.item_ids` as a configurable feature. Existing configuration and CLI toggles remain accepted as compatibility no-ops.

## Testing

- Cover default ID assignment, persistence and resume behavior, remote compaction, provider requests, WebSocket requests, and removed-feature compatibility.

GitOrigin-RevId: 7a09e2d07d2606c2ef1e8e2283d36cb555557eae

codex-rs/memories/write/src/runtime.rs

@@ -260,7 +260,6 @@ impl MemoryStartupContext {
             config.features.enabled(Feature::EnableRequestCompression),
             config.features.enabled(Feature::RuntimeMetrics),
             /*beta_features_header*/ None,
-            config.features.enabled(Feature::ItemIds),
             /*concurrent_reasoning_summaries_enabled*/ false,
             /*attestation_provider*/ None,
             config.http_client_factory(),