fix(tests): update test assertions for null model in resume /status; drop unused import

Two integration tests expected 'model':'restored-session' in the /status
JSON output but dc4fa55 changed resume mode to emit null for model.
Updated both assertions to assert model is null (correct behavior).

Also remove unused 'estimate_session_tokens' import in compact.rs tests
(surfaced as warning in CI, kept failing CI green noise).

All workspace tests pass.

rust/crates/runtime/src/compact.rs

@@ -555,7 +555,7 @@ fn extract_summary_timeline(summary: &str) -> Vec<String> {
 #[cfg(test)]
 mod tests {
     use super::{
-        collect_key_files, compact_session, estimate_session_tokens, format_compact_summary,
+        collect_key_files, compact_session, format_compact_summary,
         get_compact_continuation_message, infer_pending_work, should_compact, CompactionConfig,
     };
     use crate::session::{ContentBlock, ConversationMessage, MessageRole, Session};