feat: generate_embedding routes non-default models through client.embed (T112.3)

When model != DEFAULT_EMBEDDING_MODEL, generate_embedding now
calls client.embed(text, model=model) and wraps the returned
vector in an EmbeddingResult tagged with the requested model.
On any exception (NotImplementedError from providers without an
embeddings endpoint, transient network errors, etc.), the existing
T107 warning fires and the function falls back to the zero-vector
sentinel — callers detect model == 'fallback' and skip indexing.

Adds:
- MockLLMClient accepts a canned_embeddings queue mirroring
  the existing canned pattern. embed() pops from the front;
  empty queue raises IndexError so misconfigured tests fail
  loudly.
- Settings.embedding_model defaults to "pseudo-sha256-384"
  so existing zero-config installs keep Phase 4 behavior. The app
  lifespan now passes this through to EmbeddingWorker.model.

The public signature of generate_embedding is unchanged:
(client, *, text, model=DEFAULT_EMBEDDING_MODEL, dim=..., timeout_s=...).

tests/test_llm_mock.py

@@ -19,3 +19,28 @@ async def test_mock_streams_tokens():
     async for chunk in client.stream(msgs, model="any"):
         chunks.append(chunk)
     assert "".join(chunks) == "abcd"
+
+
+@pytest.mark.asyncio
+async def test_mock_llm_client_embed_pops_canned():
+    """T112: MockLLMClient.embed() pops a canned vector from the front
+    of ``canned_embeddings`` (mirrors the existing ``canned`` queue
+    pattern for generate/stream)."""
+    v1 = [0.1, 0.2, 0.3]
+    v2 = [0.4, 0.5, 0.6]
+    client = MockLLMClient(canned=[], canned_embeddings=[v1, v2])
+
+    out1 = await client.embed("first", model="bge-small-en-v1.5")
+    out2 = await client.embed("second", model="bge-small-en-v1.5")
+    assert out1 == v1
+    assert out2 == v2
+
+
+@pytest.mark.asyncio
+async def test_mock_llm_client_embed_empty_queue_raises():
+    """When the canned_embeddings queue is empty, ``embed`` must raise
+    a clear failure (IndexError) so misconfigured tests don't silently
+    return None or hang."""
+    client = MockLLMClient(canned=[])
+    with pytest.raises(IndexError):
+        await client.embed("text", model="any")