fix(session): cap messages at persistence boundary

Bind SessionManager saves to the existing raw archive path so SDK imports and other bypass saves cannot persist more than the file cap without archiving unconsolidated overflow.

Add an SDK regression test that exercises the real ingest path.

Refs #4787

nanobot/session/manager.py

@@ -12,7 +12,7 @@ from copy import deepcopy
 from dataclasses import dataclass, field
 from datetime import datetime
 from pathlib import Path
-from typing import Any
+from typing import Any, Callable
 from weakref import WeakValueDictionary
 
 from loguru import logger
@@ -427,6 +427,7 @@ class SessionManager:
         # Preserve identity for sessions held by active callers without retaining idle ones.
         self._overflow_cache: WeakValueDictionary[str, Session] = WeakValueDictionary()
         self._max_cached_sessions = SESSION_CACHE_MAX_SIZE
+        self._file_cap_archiver: Callable[..., None] | None = None
 
     def _remember(self, session: Session) -> None:
         """Keep recent sessions strongly cached without duplicating live objects."""
@@ -448,6 +449,10 @@ class SessionManager:
             self._remember(session)
         return session
 
+    def set_file_cap_archiver(self, archiver: Callable[..., None]) -> None:
+        """Archive unconsolidated overflow whenever a session is persisted."""
+        self._file_cap_archiver = archiver
+
     @staticmethod
     def safe_key(key: str) -> str:
         """Public helper used by HTTP handlers to map an arbitrary key to a stable filename stem."""
@@ -669,6 +674,14 @@ class SessionManager:
         write-back caching (e.g. rclone VFS, NFS, FUSE mounts) do not lose
         the most recent writes.
         """
+        if self._file_cap_archiver is not None:
+            session.enforce_file_cap(
+                on_archive=lambda messages: self._file_cap_archiver(
+                    messages,
+                    session_key=session.key,
+                )
+            )
+
         path = self._get_session_path(session.key)
         tmp_path = path.with_suffix(".jsonl.tmp")