Omit service_tier from remote /responses/compact requests under API auth (#21676)

## Summary

API-key-auth remote compaction requests should not inherit
`service_tier` from normal `/responses` turns. This path needs to match
API auth expectations, while ChatGPT-auth remote compaction should keep
reusing the shared request fields that still apply there.

This change keeps the decision inline in
`codex-rs/core/src/compact_remote.rs` only. Under API key auth, the
classic remote `/responses/compact` path now omits `service_tier`; under
ChatGPT auth, it keeps reusing the configured tier.
`codex-rs/core/src/compact_remote_v2.rs` is unchanged. The remote
compaction parity coverage and snapshots were updated to assert the
API-key omission and preserve the ChatGPT-auth behavior.

## Testing

- Updated remote compaction parity coverage in
`codex-rs/core/tests/suite/compact_remote.rs` and the corresponding
snapshots.

codex-rs/core/src/compact_tests.rs

@@ -208,7 +208,6 @@ fn should_use_remote_compact_task_for_azure_provider() {
 
     assert!(should_use_remote_compact_task(&provider));
 }
-
 #[tokio::test]
 async fn process_compacted_history_replaces_developer_messages() {
     let compacted_history = vec![