feat: add remote compaction v2 Responses client path (#20773)

## Why

This adds the `remote_compaction_v2` client path so remote compaction
can run through the normal Responses stream and install a
`context_compaction` item that trigger a compaction.

The goal is to migrate some of the compaction logic on the client side

We keeps the v2 transport behind a feature flag while letting follow-up
requests reuse the compacted context instead of falling back to the
legacy compaction item shape.

## What changed

- add `ResponseItem::ContextCompaction` and refresh the generated
app-server / schema / TypeScript fixtures that expose response items on
the wire
- add `core/src/compact_remote_v2.rs` to send compaction through the
standard streamed Responses client, require exactly one
`context_compaction` output item, and install that item into compacted
history
- route manual compact and auto-compaction through the v2 path when
`remote_compaction_v2` is enabled, while keeping the existing remote
compaction path as the fallback
- preserve the new item type across history retention, follow-up request
construction, telemetry, rollout persistence, and rollout-trace
normalization
- add targeted coverage for the feature flag, `context_compaction`
serialization, rollout-trace normalization, and remote-compaction
follow-up behavior

## Verification

- added protocol tests for `context_compaction`
serialization/deserialization in `protocol/src/models.rs`
- added rollout-trace coverage for `context_compaction` normalization in
`rollout-trace/src/reducer/conversation_tests.rs`
- added remote compaction integration coverage for v2 follow-up reuse
and mixed compaction output streams in
`core/tests/suite/compact_remote.rs`

---------

Co-authored-by: Codex <noreply@openai.com>

codex-rs/features/src/tests.rs

@@ -119,6 +119,16 @@ fn request_permissions_tool_is_under_development() {
     assert_eq!(Feature::RequestPermissionsTool.default_enabled(), false);
 }
 
+#[test]
+fn remote_compaction_v2_is_under_development() {
+    assert_eq!(Feature::RemoteCompactionV2.stage(), Stage::UnderDevelopment);
+    assert_eq!(Feature::RemoteCompactionV2.default_enabled(), false);
+    assert_eq!(
+        feature_for_key("remote_compaction_v2"),
+        Some(Feature::RemoteCompactionV2)
+    );
+}
+
 #[test]
 fn terminal_resize_reflow_is_experimental_and_enabled_by_default() {
     assert_eq!(