feat(agent): make model presets session-scoped (#4866)

nanobot/session/model_selection.py

@@ -0,0 +1,20 @@
+"""Session-scoped model preset metadata."""
+
+from __future__ import annotations
+
+from collections.abc import Mapping
+
+# Session.metadata is public SDK data, so internal selectors use a reserved namespace.
+SESSION_MODEL_PRESET_METADATA_KEY = "_nanobot_model_preset"
+
+
+def model_preset_from_metadata(metadata: object) -> str | None:
+    """Read the canonical session preset name from persisted metadata."""
+    if not isinstance(metadata, Mapping):
+        return None
+    if SESSION_MODEL_PRESET_METADATA_KEY not in metadata:
+        return None
+    value = metadata[SESSION_MODEL_PRESET_METADATA_KEY]
+    if not isinstance(value, str) or not value.strip():
+        raise ValueError("session model preset must be a non-empty string")
+    return value.strip()