feat(rag): 新增 Embedder 抽象与离线 FakeEmbedder

tests/test_rag_embeddings.py

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+# tests/test_rag_embeddings.py
+from genesis.rag.embeddings import FakeEmbedder, get_embedder
+
+
+def test_fake_embedder_deterministic():
+    e = FakeEmbedder()
+    a = e.embed(["OrderController 创建订单"])[0]
+    b = e.embed(["OrderController 创建订单"])[0]
+    assert a == b
+
+
+def test_fake_embedder_similar_closer_than_unrelated():
+    e = FakeEmbedder()
+    base = e.embed(["OrderController 处理创建订单请求"])[0]
+    sim = e.embed(["OrderController 保存订单到数据库"])[0]
+    dif = e.embed(["用户登录认证模块"])[0]
+    import math
+
+    def cos(x, y):
+        dot = sum(p * q for p, q in zip(x, y))
+        nx = math.sqrt(sum(p * p for p in x))
+        ny = math.sqrt(sum(q * q for q in y))
+        return dot / (nx * ny or 1.0)
+
+    assert cos(base, sim) > cos(base, dif)
+
+
+def test_get_embedder_fake_engine_returns_fake():
+    e = get_embedder("fake")
+    assert e.__class__.__name__ == "FakeEmbedder"