get_artifacts returns small bytes artifacts as {'__bytes__': ...} dicts #536

Bug

ProgramDatabase.get_artifacts() returns small bytes artifacts (under the 32KB disk threshold) as {'__bytes__': '<base64>'} dicts instead of bytes, so evaluators that return binary artifacts get the encoded dict back and it ends up in prompts as-is.

Reproduction

import tempfile
from openevolve.config import DatabaseConfig
from openevolve.database import Program, ProgramDatabase

db = ProgramDatabase(DatabaseConfig(db_path=tempfile.mkdtemp()))
db.add(Program(id="p1", code="print('hello')", metrics={"score": 0.5}))
db.store_artifacts("p1", {"blob": b"\x00\x01binary"})
print(db.get_artifacts("p1"))

Expected: {'blob': b'\x00\x01binary'} Actual: {'blob': {'__bytes__': 'AAFiaW5hcnk='}}

Large artifacts written to disk already come back as bytes, so the return type currently depends on the artifact's size.

Cause

store_artifacts encodes with json.dumps(..., default=self._artifact_serializer) (openevolve/database.py:2633), but get_artifacts decodes with a plain json.loads (openevolve/database.py:2663) and never uses the matching _artifact_deserializer, which is otherwise unused.

Fix

A minimal fix with a regression test is in the linked PR.

I found this with patchwork, an automated code-audit tool I run, and verified the reproduction test in an isolated sandbox. If tool-assisted reports aren't welcome here, let me know and I won't send more.