test(evidence): harden generation lifecycle boundaries

harness/tht/adapters/vector/pgvector.py

@@ -405,7 +405,7 @@ class PgVectorStore:
         return len(records)
 
     def delete_generation(self, collection: str, generation: str) -> int:
-        if collection != "evidence" or not generation.startswith("gen:"):
+        if collection != "evidence" or re.fullmatch(r"gen:[0-9a-f]{32}", generation) is None:
             raise VectorStoreError("Only exact Evidence generations may be deleted")
         raw = None
         try:

harness/tht/adapters/vector/thoth_http.py

@@ -1,5 +1,7 @@
 """Thoth vector HTTP adapter using distinct read and write clients."""
 
+import re
+
 from tht.ports.vector import (
     VectorCapabilities,
     VectorHealth,
@@ -154,7 +156,7 @@ class ThothHttpVectorStore:
             raise VectorStoreError(str(exc)) from exc
 
     def delete_generation(self, collection: str, generation: str) -> int:
-        if collection != "evidence" or not generation.startswith("gen:"):
+        if collection != "evidence" or re.fullmatch(r"gen:[0-9a-f]{32}", generation) is None:
             raise VectorStoreError("Only exact Evidence generations may be deleted")
         try:
             return self._require_writer().delete_generation(collection, generation)