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/factories.py

@@ -0,0 +1,86 @@
+"""Test data builders for preprocessing and dataset scenarios."""
+
+from astrai.config.preprocess_config import (
+    InputConfig,
+    PipelineConfig,
+    ProcessingConfig,
+)
+
+CHAT_SECTIONS = [{"field": "messages", "action": "$role", "template": True}]
+INSTRUCTION_SECTIONS = [
+    {"field": "prompt", "action": "mask", "add_special_tokens": True},
+    {"field": "response", "action": "train"},
+]
+TEXT_SECTIONS = [{"field": "text", "action": "train"}]
+GRPO_RESPONSE_SECTIONS = [{"field": "responses", "action": "train"}]
+
+
+def make_pipeline_config(sections, *, mask=None, preprocessing=None, sources=None):
+    """Build a pipeline config with the common test defaults."""
+    return PipelineConfig(
+        input=InputConfig(sections=sections, sources=sources),
+        mask={} if mask is None else mask,
+        mask_default="mask",
+        preprocessing=preprocessing or ProcessingConfig(max_seq_len=2048),
+    )
+
+
+def make_chat_config():
+    return make_pipeline_config(
+        CHAT_SECTIONS,
+        mask={"system": "mask", "user": "mask", "assistant": "train"},
+    )
+
+
+def make_instruction_config():
+    return make_pipeline_config(
+        INSTRUCTION_SECTIONS,
+        mask={"prompt": "mask", "response": "train"},
+    )
+
+
+def make_text_config():
+    return make_pipeline_config(
+        TEXT_SECTIONS,
+        preprocessing=ProcessingConfig(
+            max_seq_len=2048, min_chars=1, max_chars=2_000_000
+        ),
+    )
+
+
+def make_dpo_chat_config():
+    sources = {
+        name: {"sections": [{"field": name, "action": "$role", "template": True}]}
+        for name in ("chosen", "rejected")
+    }
+    return make_pipeline_config(
+        None,
+        mask={"user": "mask", "assistant": "train"},
+        sources=sources,
+    )
+
+
+def make_grpo_config(*, template=True):
+    prompt_section = {"field": "prompt", "action": "mask"}
+    if template:
+        prompt_section["template"] = True
+    else:
+        prompt_section["add_special_tokens"] = True
+    sources = {
+        "prompts": {"sections": [prompt_section]},
+        "responses": {
+            "sections": GRPO_RESPONSE_SECTIONS,
+            "list_field": True,
+            "mask_key": "masks",
+        },
+        "rewards": {"sections": [{"field": "rewards", "action": "value"}]},
+    }
+    return make_pipeline_config(
+        None,
+        mask={"user": "mask", "assistant": "train"},
+        sources=sources,
+    )
+
+
+def make_grpo_no_template_config():
+    return make_grpo_config(template=False)