refactor: move exception subclasses to phase-3 where they are used

FileStorageError, RayActorError, ToolExecutionError, and UnexpectedError
are not raised in this phase — they belong in phase-3 (core services).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: paultranvan

openrag/utils/exceptions/__init__.py

@@ -1,2 +1 @@
 from .base import *
-from .common import *