core: enable remote compaction v2 by default (#27573)

## Why

Remote compaction v2 is ready to become the default for providers that
already support remote compaction. Leaving it behind an
under-development opt-in keeps eligible sessions on the legacy
remote-compaction path.

This does not broaden provider eligibility: OpenAI and Azure move to v2,
while Bedrock and OSS providers retain their existing local-compaction
behavior.

## What changed

- Mark `remote_compaction_v2` stable and enable it by default.
- Make tests that intentionally cover legacy remote compaction
explicitly disable v2.
- Update parity coverage so v2 exercises the production default and only
legacy mode opts out.

## Verification

- `just test -p codex-core
auto_compact_runs_after_resume_when_token_usage_is_over_limit
auto_compact_counts_encrypted_reasoning_before_last_user
auto_compact_runs_when_reasoning_header_clears_between_turns
responses_lite_compact_request_uses_lite_transport_contract`

codex-rs/features/src/tests.rs

@@ -150,16 +150,6 @@ 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!(