fix(embeddings): survive numpy embeddings when restoring a reset lane (#3410)

When a lane reset fails to rewrite the recreated collection, the recovery path
re-adds the preserved rows. It read the embeddings with
`preserved.get("embeddings") or []` and gated the loop with
`if ids and docs and old_embeddings:`. chromadb returns embeddings as a numpy
ndarray, whose truth value is ambiguous, so both expressions raise ValueError
inside the except block — the restore is abandoned and every preserved row is
lost (the collection was already deleted), exactly when the code is trying to
avoid data loss.

Use an explicit `is None` check and `len(...)`, and convert ndarray batches to
lists before re-adding.

Adds tests/test_embedding_lane_ndarray_restore.py (preserved embeddings come
back as np.ndarray); existing test_embedding_lanes.py still passes.

tests/test_embedding_lane_ndarray_restore.py

@@ -0,0 +1,68 @@
+"""Embedding-lane reset must restore rows even when chromadb returns the
+preserved embeddings as a numpy ndarray.
+
+Real chromadb returns collection.get(include=["embeddings"]) as a numpy
+ndarray. The restore-after-failed-rewrite path used `embeddings or []` and a
+bare `if ... and embeddings:`, both of which raise
+"truth value of an array ... is ambiguous" on an ndarray — aborting the
+restore and wiping the collection the reset was meant to preserve.
+
+This mirrors test_lane_reset_restores_existing_collection_when_rewrite_fails
+in test_embedding_lanes.py, but the preserved embeddings come back as ndarray.
+"""
+import numpy as np
+
+from src.embedding_lanes import build_embedding_lanes
+from tests.test_embedding_lanes import FakeChroma, FakeEmbedder, _patch_chroma
+
+
+def test_lane_reset_restores_when_chroma_returns_numpy_embeddings(monkeypatch):
+    fake = FakeChroma()
+    old_custom = fake.get_or_create_collection(
+        "odysseus_memories_custom",
+        metadata={
+            "embedding_lane": "custom",
+            "embedding_dimension": 384,
+            "embedding_fingerprint": "old",
+        },
+    )
+    old_custom.add(
+        ids=["existing-memory"],
+        embeddings=[[0.0] * 384],
+        documents=["existing custom memory"],
+        metadatas=[{"source": "memory"}],
+    )
+
+    # Make the preserved embeddings come back as a numpy ndarray, like real
+    # chromadb does.
+    real_get = old_custom.get
+
+    def ndarray_get(*args, **kwargs):
+        result = real_get(*args, **kwargs)
+        result["embeddings"] = np.array(result["embeddings"])
+        return result
+
+    old_custom.get = ndarray_get
+
+    # Force the post-reset rewrite to fail so the restore branch runs.
+    fake.fail_next_add_for["odysseus_memories_custom"] = 1
+    _patch_chroma(monkeypatch, fake)
+
+    import src.embedding_lanes as lanes
+
+    monkeypatch.setattr(lanes, "_build_custom_client", lambda: FakeEmbedder(768, "nomic", "http://embeddings/v1"))
+
+    def fail_fastembed():
+        raise RuntimeError("fastembed missing")
+
+    monkeypatch.setattr(lanes, "_build_fastembed_client", fail_fastembed)
+
+    built = build_embedding_lanes("odysseus_memories")
+
+    # Both lanes are unavailable, but the existing row must survive — not be
+    # wiped by an ndarray-truthiness crash in the restore path.
+    assert built == []
+    restored = fake.collections["odysseus_memories_custom"]
+    assert restored.count() == 1
+    assert restored.get()["ids"] == ["existing-memory"]
+    assert len(restored.rows["existing-memory"]["embedding"]) == 384