fix(kb): scope generation lock by doc_id/parse_hash and surface embed cleanup failures (PR #202)

Replace source_path ingestion locks with cross-process generation_ingestion_lock
keyed by (collection, doc_id, parse_hash, user scope), held through chunk replace
and embedding writes. Raise DatabaseOperationError when embeddings_* cascade
delete fails. Add regression tests for empty replace_scope, partial cleanup
failures, and per-user lock isolation.

Refs: #199, #438

Author: sqhyz55

src/xagent/core/tools/core/RAG_tools/chunk/chunk_document.py

@@ -88,10 +88,10 @@ def chunk_document(
     Chunk parsed paragraphs and write to chunks table.
 
     Concurrency note:
-        This function uses **last-write-wins** semantics. The ingestion
-        pipeline serialises calls per (collection, source_path) via
-        ``_get_ingestion_lock``, so concurrent chunk replacement races are
-        prevented at the pipeline level.
+        Chunk replacement is serialised per
+        ``(collection, doc_id, parse_hash, user scope)`` via
+        ``generation_ingestion_lock`` (in ``replace_chunks`` and the ingestion
+        pipeline through embedding writes).
 
     Args:
         collection: Collection name for data isolation