[codex] Use compaction_trigger item for remote compaction v2 (#22809)

## Why

Remote compaction v2 was still using `context_compaction` as both the
request trigger and the compacted output shape. The Responses API now
has the landed contract for this flow: Codex sends a dedicated `{
"type": "compaction_trigger" }` input item, and the backend returns the
standard `compaction` output item with encrypted content.

This aligns the v2 path with that wire contract while preserving the
existing local compacted-history post-processing behavior.

## What changed

- Add `ResponseItem::CompactionTrigger` and regenerate the app-server
protocol schema fixtures.
- Send `compaction_trigger` from `remote_compaction_v2` instead of a
payload-less `context_compaction`.
- Collect exactly one backend `compaction` output item, then reuse the
existing compacted-history rebuilding path.
- Treat the trigger item as a transient request marker rather than model
output or persisted rollout/memory content.

## Verification

- `cargo test -p codex-protocol compaction_trigger`
- `cargo test -p codex-core remote_compact_v2`
- `cargo test -p codex-core compact_remote_v2`
- `cargo test -p codex-core
responses_websocket_sends_response_processed_after_remote_compaction_v2`
- `just write-app-server-schema`
- `cargo test -p codex-app-server-protocol schema_fixtures`

codex-rs/app-server-protocol/schema/json/codex_app_server_protocol.schemas.json

@@ -14065,6 +14065,22 @@
             "title": "CompactionResponseItem",
             "type": "object"
           },
+          {
+            "properties": {
+              "type": {
+                "enum": [
+                  "compaction_trigger"
+                ],
+                "title": "CompactionTriggerResponseItemType",
+                "type": "string"
+              }
+            },
+            "required": [
+              "type"
+            ],
+            "title": "CompactionTriggerResponseItem",
+            "type": "object"
+          },
           {
             "properties": {
               "encrypted_content": {