fix(audit): _collect_block_sources mirrors None-doc-types (provenance accuracy, FU-7 review)

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

mcp-server/src/legal_mcp/services/block_writer.py

@@ -421,10 +421,13 @@ async def _collect_block_sources(case_id: UUID, block_id: str) -> dict:
     claim_ids: extracted claims for the case. (case_law_ids are captured
     separately from the precedent search inside write_block.)
     """
-    allowed = _BLOCK_DOC_TYPES.get(block_id, [])
-    docs = await db.list_documents(case_id)
-    if allowed:
-        docs = [d for d in docs if d.get("doc_type") in allowed]
+    allowed = _BLOCK_DOC_TYPES.get(block_id, [])  # [] = all docs; None = no source docs
+    if allowed is None:
+        docs = []  # mirror _build_source_context: this block consumes no raw source docs
+    else:
+        docs = await db.list_documents(case_id)
+        if allowed:
+            docs = [d for d in docs if d.get("doc_type") in allowed]
     claims = await db.get_claims(case_id)
     return {
         "document_ids": [str(d["id"]) for d in docs],