refactor(session): return RetentionResult instead of bare tuple

Replace the tuple(list[dict], int) return of
Session.retain_recent_legal_suffix with a named RetentionResult
dataclass that exposes retained, dropped,
already_consolidated_count, and new_last_consolidated fields.

The tuple return was easy to misuse because the second value only
made sense relative to the first and the old last_consolidated
cursor. The named fields make the archive-skip semantics explicit
at every call site.

No behavior change. All existing tests pass unchanged in semantics.

Refs #4136

Signed-off-by: axelray-dev <110029405+axelray-dev@users.noreply.github.com>

tests/agent/test_auto_compact.py

@@ -103,12 +103,12 @@ def _make_fake_compact(
             metadata={},
             last_consolidated=0,
         )
-        dropped, already_consolidated = probe.retain_recent_legal_suffix(
+        result = probe.retain_recent_legal_suffix(
             max_suffix,
             extend_to_user=True,
         )
         kept = probe.messages
-        archive_msgs = dropped[already_consolidated:]
+        archive_msgs = result.dropped[result.already_consolidated_count:]
 
         if not archive_msgs and not kept:
             session.updated_at = datetime.now()