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_preprocess_pipeline.py

@@ -9,10 +9,10 @@ from astrai.config.preprocess_config import (
 )
 from astrai.preprocessing.packing import PackingStrategyFactory
 from astrai.preprocessing.pipeline import Pipeline, filter_by_length
-from tests.data.conftest import (
-    _CHAT_SECTIONS,
-    _INSTRUCTION_SECTIONS,
-    _TEXT_SECTIONS,
+from tests.data.factories import (
+    CHAT_SECTIONS,
+    INSTRUCTION_SECTIONS,
+    TEXT_SECTIONS,
     make_dpo_chat_config,
     make_grpo_no_template_config,
 )
@@ -54,7 +54,7 @@ def test_full_chat_pipeline(temp_dir, chat_tokenizer_dir):
         )
 
     config = PipelineConfig(
-        input=InputConfig(sections=_CHAT_SECTIONS),
+        input=InputConfig(sections=CHAT_SECTIONS),
         mask={"system": "mask", "user": "mask", "assistant": "train"},
         mask_default="mask",
         preprocessing=ProcessingConfig(max_seq_len=2048),
@@ -97,7 +97,7 @@ def test_full_text_pipeline(temp_dir, tokenizer_dir):
         )
 
     config = PipelineConfig(
-        input=InputConfig(sections=_TEXT_SECTIONS),
+        input=InputConfig(sections=TEXT_SECTIONS),
         preprocessing=ProcessingConfig(max_seq_len=2048, min_chars=10),
         output=OutputConfig(storage_format="bin"),
     )
@@ -138,7 +138,7 @@ def test_full_instruction_pipeline(temp_dir, tokenizer_dir):
         )
 
     config = PipelineConfig(
-        input=InputConfig(sections=_INSTRUCTION_SECTIONS),
+        input=InputConfig(sections=INSTRUCTION_SECTIONS),
         mask={"prompt": "mask", "response": "train"},
         mask_default="mask",
         preprocessing=ProcessingConfig(max_seq_len=2048),
@@ -164,7 +164,7 @@ def test_dtype_override(temp_dir, tokenizer_dir):
         f.write(json.dumps({"prompt": "Q", "response": "A"}) + "\n")
 
     config = PipelineConfig(
-        input=InputConfig(sections=_INSTRUCTION_SECTIONS),
+        input=InputConfig(sections=INSTRUCTION_SECTIONS),
         mask={"prompt": "mask", "response": "train"},
         mask_default="mask",
         preprocessing=ProcessingConfig(max_seq_len=2048),