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_config.py

@@ -24,3 +24,25 @@ def test_chat_db_path_env_overrides_default(tmp_path, monkeypatch):
     (tmp_path / "config.toml").write_text('featherless_api_key = "x"\n')
     s = load_settings()
     assert s.db_path == tmp_path / "alt.db"
+
+
+def test_embedding_model_defaults_to_pseudo(tmp_path, monkeypatch):
+    """T112: ``embedding_model`` defaults to the deterministic pseudo
+    so existing zero-config installs keep the Phase 4 behavior."""
+    monkeypatch.setenv("CHAT_CONFIG_PATH", str(tmp_path / "config.toml"))
+    (tmp_path / "config.toml").write_text('featherless_api_key = "x"\n')
+    s = load_settings()
+    assert s.embedding_model == "pseudo-sha256-384"
+
+
+def test_embedding_model_overridable_via_toml(tmp_path, monkeypatch):
+    """T112: operators swap the embedding model by editing config.toml.
+    The new value flows through to the embedding worker at startup."""
+    cfg = tmp_path / "config.toml"
+    cfg.write_text(
+        'featherless_api_key = "x"\n'
+        'embedding_model = "bge-small-en-v1.5"\n'
+    )
+    monkeypatch.setenv("CHAT_CONFIG_PATH", str(cfg))
+    s = load_settings()
+    assert s.embedding_model == "bge-small-en-v1.5"