perf(retrieval): one embedding per query per plugin request (milestone 456 step 1, #4903)

A single plugin request fans one query out to several ranked arms. The
operator message is searched by auto_inject, its reuse and lesson slots,
prompt_rule, the preference slot and rule_via_lesson, and each arm called
get_embedding on its own: up to six model calls for one vector.

- embeddings.query_embedding_memo(): a request-scoped ContextVar memo that
  get_embedding consults. Outside a scope the default is None, so every
  other caller is unchanged. A failed embedding is not remembered.
- memoized_query_embeddings decorates /retrieve, /tool-rules and
  /prior-art.
- get_embeddings (document chunks) is untouched.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/test_embeddings.py

@@ -72,3 +72,102 @@ async def test_get_embedding_propagates_model_load_failures():
     ):
         with pytest.raises(RuntimeError, match="model load failed"):
             await get_embedding("x")
+
+
+# ─── query_embedding_memo (milestone 456 step 1) ─────────────────────────────
+
+
+def _counting_embeddings():
+    """A stand-in for get_embeddings that counts model calls and returns a
+    vector derived from the text, so a wrong memo key returns a wrong vector."""
+    calls: list[list[str]] = []
+
+    async def fake(texts):
+        calls.append(list(texts))
+        return [[float(len(t)), 1.0] for t in texts]
+
+    return fake, calls
+
+
+@pytest.mark.asyncio
+async def test_memo_embeds_a_repeated_query_once_per_scope():
+    from scribe.services import embeddings as emb
+
+    fake, calls = _counting_embeddings()
+    with patch.object(emb, "get_embeddings", fake):
+        with emb.query_embedding_memo():
+            first = await emb.get_embedding("push to dev")
+            again = await emb.get_embedding("push to dev")
+            other = await emb.get_embedding("merge to main")
+    assert first == again == [11.0, 1.0]
+    assert other == [13.0, 1.0]
+    assert calls == [["push to dev"], ["merge to main"]]
+
+
+@pytest.mark.asyncio
+async def test_without_a_scope_every_call_reaches_the_model():
+    """The default is None: outside a request scope nothing is remembered, so
+    every caller other than the plugin routes behaves exactly as before."""
+    from scribe.services import embeddings as emb
+
+    fake, calls = _counting_embeddings()
+    with patch.object(emb, "get_embeddings", fake):
+        await emb.get_embedding("q")
+        await emb.get_embedding("q")
+    assert calls == [["q"], ["q"]]
+
+
+@pytest.mark.asyncio
+async def test_a_scope_does_not_leak_into_the_next_one():
+    from scribe.services import embeddings as emb
+
+    fake, calls = _counting_embeddings()
+    with patch.object(emb, "get_embeddings", fake):
+        with emb.query_embedding_memo():
+            await emb.get_embedding("q")
+        with emb.query_embedding_memo():
+            await emb.get_embedding("q")
+        await emb.get_embedding("q")
+    assert calls == [["q"], ["q"], ["q"]]
+    assert emb._query_memo.get() is None
+
+
+@pytest.mark.asyncio
+async def test_a_failed_embedding_is_not_remembered():
+    """The next arm in the request must try again and degrade on its own,
+    not inherit a failure as if it were a vector."""
+    from scribe.services import embeddings as emb
+
+    attempts = {"n": 0}
+
+    async def flaky(texts):
+        attempts["n"] += 1
+        if attempts["n"] == 1:
+            raise RuntimeError("model busy")
+        return [[0.5, 0.5] for _ in texts]
+
+    with patch.object(emb, "get_embeddings", flaky):
+        with emb.query_embedding_memo():
+            with pytest.raises(RuntimeError):
+                await emb.get_embedding("q")
+            assert await emb.get_embedding("q") == [0.5, 0.5]
+    assert attempts["n"] == 2
+
+
+@pytest.mark.asyncio
+async def test_the_route_decorator_opens_one_scope_per_call():
+    from scribe.services import embeddings as emb
+
+    fake, calls = _counting_embeddings()
+
+    @emb.memoized_query_embeddings
+    async def handler(q):
+        await emb.get_embedding(q)
+        await emb.get_embedding(q)
+        return emb._query_memo.get() is not None
+
+    with patch.object(emb, "get_embeddings", fake):
+        assert await handler("a") is True
+        assert await handler("a") is True
+    assert calls == [["a"], ["a"]]
+    assert handler.__name__ == "handler"