refactor: split mask builder by single/multi output

- Extract SingleOutputMaskBuilder for SFT and pretrain configs
- Extract MultiOutputMaskBuilder for DPO and GRPO configs
- Keep SectionedMaskBuilder as backward-compatible facade
- Register "single" and "multi" names in MaskBuilderFactory
- Add parity and rejection tests for concrete builders

tests/data/conftest.py

@@ -10,7 +10,11 @@ from astrai.config.preprocess_config import (
     PipelineConfig,
     ProcessingConfig,
 )
-from astrai.preprocessing.builder import SectionedMaskBuilder
+from astrai.preprocessing.builder import (
+    MultiOutputMaskBuilder,
+    SectionedMaskBuilder,
+    SingleOutputMaskBuilder,
+)
 from astrai.tokenize import AutoTokenizer
 
 _SPECIAL_TOKENS_CONFIG = {
@@ -210,6 +214,16 @@ def builder():
     return SectionedMaskBuilder()
 
 
+@pytest.fixture
+def single_builder():
+    return SingleOutputMaskBuilder()
+
+
+@pytest.fixture
+def multi_builder():
+    return MultiOutputMaskBuilder()
+
+
 @pytest.fixture
 def tokenizer_dir(temp_dir, test_tokenizer):
     d = os.path.join(temp_dir, "tok")