Always use streamed remote compaction for supported providers (#44255)

## What changed

- Route manual and automatic remote compaction through the v2 implementation.
- Retire the `remote_compaction_v2` feature toggle while continuing to advertise it in the model client beta features header.
- Migrate compaction tests from `/responses/compact` to streamed `/responses` requests with `compaction_trigger` items and remove legacy parity coverage.

## Testing

Add streamed compaction coverage for tool search output trimming and session base instruction token estimates. Update app-server tests to verify started/completed notifications for both local and streamed remote compaction.

GitOrigin-RevId: cea392a6166a9b116ff83708fd9126d5546476e4

codex-rs/core/src/lib.rs

@@ -29,6 +29,10 @@ pub use responses_metadata::CodexResponsesMetadata;
 pub use turn_metadata::detached_memory_responses_metadata;
 mod codex_thread;
 mod compact_model_fallback;
+#[expect(
+    dead_code,
+    reason = "the unrouted v1 runner is removed in the follow-up"
+)]
 mod compact_remote;
 mod compact_remote_history;
 mod compact_remote_v2;