fix(core): validate file existence before creating files_agents rows (#9341)

Prevents ForeignKeyViolationError when attaching files to agents where
the file has been deleted between listing and attachment (race condition).
Now validates file IDs exist in the files table before inserting, and
skips any missing files with a warning log.

Fixes Datadog issue a1768774-d691-11f0-9330-da7ad0900000

🐾 Generated with [Letta Code](https://letta.com)

Co-authored-by: Letta <noreply@letta.com>

letta/services/files_agents_manager.py

@@ -5,6 +5,7 @@ from sqlalchemy import and_, delete, func, or_, select, update
 
 from letta.log import get_logger
 from letta.orm.errors import NoResultFound
+from letta.orm.file import FileMetadata as FileMetadataModel
 from letta.orm.files_agents import FileAgent as FileAgentModel
 from letta.otel.tracing import trace_method
 from letta.schemas.block import Block as PydanticBlock, FileBlock as PydanticFileBlock
@@ -696,6 +697,20 @@ class FileAgentManager:
                 closed_file_names.extend(new_names[max_files_open:])
             evicted_ids = [r.file_id for r in currently_open if r.file_name in closed_file_names]
 
+            # validate file IDs exist to prevent FK violations (files may have been deleted)
+            requested_file_ids = {meta.id for meta in ordered_unique}
+            existing_file_ids_q = select(FileMetadataModel.id).where(FileMetadataModel.id.in_(requested_file_ids))
+            existing_file_ids = set((await session.execute(existing_file_ids_q)).scalars().all())
+            missing_file_ids = requested_file_ids - existing_file_ids
+            if missing_file_ids:
+                logger.warning(
+                    "attach_files_bulk: skipping %d file(s) with missing records for agent %s: %s",
+                    len(missing_file_ids),
+                    agent_id,
+                    missing_file_ids,
+                )
+                ordered_unique = [m for m in ordered_unique if m.id in existing_file_ids]
+
             # upsert requested files
             for meta in ordered_unique:
                 is_now_open = meta.file_name in final_open_set