feat(memory): support multi-vendor embedding fallback

Add embedding_provider config knob with native support for
openai / dashscope / doubao / zhipu / linkai, plus an in-chat
/memory status and /memory rebuild-index workflow for switching
vendors safely.

agent/memory/embedding/__init__.py

@@ -0,0 +1,41 @@
+"""
+Embedding subsystem for memory.
+
+Public API:
+  create_embedding_provider, EmbeddingProvider, OpenAIEmbeddingProvider,
+  EMBEDDING_VENDORS, EmbeddingCache
+  RebuildResult, clear_index, rebuild_in_process
+  detect_index_dim, cleanup_legacy_state_file
+"""
+
+from agent.memory.embedding.provider import (
+    EMBEDDING_VENDORS,
+    DoubaoEmbeddingProvider,
+    EmbeddingCache,
+    EmbeddingProvider,
+    OpenAIEmbeddingProvider,
+    create_embedding_provider,
+)
+from agent.memory.embedding.rebuild import (
+    RebuildResult,
+    clear_index,
+    rebuild_in_process,
+)
+from agent.memory.embedding.state import (
+    cleanup_legacy_state_file,
+    detect_index_dim,
+)
+
+__all__ = [
+    "EMBEDDING_VENDORS",
+    "DoubaoEmbeddingProvider",
+    "EmbeddingCache",
+    "EmbeddingProvider",
+    "OpenAIEmbeddingProvider",
+    "create_embedding_provider",
+    "RebuildResult",
+    "clear_index",
+    "rebuild_in_process",
+    "cleanup_legacy_state_file",
+    "detect_index_dim",
+]