refactor: deduplicate and restructure test suite

- extract preprocessing config factories into tests/data/factories.py
- keep conftest.py fixtures-only; stop importing builders from it
- promote temp_dir fixture to root conftest for cross-directory reuse
- unify duplicate BPE tokenizer builders into build_test_tokenizer
- merge grpo/dpo online e2e tests into one parametrized integration test
- extract engine mock factory and shared model batch builders
- drop local tempfile usage in favor of shared fixtures

No behavior change: 519 tests pass.

tests/data/test_dataset.py

@@ -24,7 +24,7 @@ from astrai.serialization import (
     load_bin,
     save_bin,
 )
-from tests.data.conftest import make_grpo_no_template_config
+from tests.data.factories import make_grpo_config
 
 
 def _rand_seq(length, vocab=1000):
@@ -797,7 +797,7 @@ def test_grpo_builder_preserves_response_boundaries(base_test_env):
     _save_test_tokenizer(base_test_env["test_dir"], tokenizer)
 
     builder = SectionedMaskBuilder()
-    config = make_grpo_no_template_config()
+    config = make_grpo_config(template=False)
     config.preprocessing.max_seq_len = 128
 
     item = {