feat: require date_column in training parameters and update documentation

- Made `date_column` a required field in `TrainModelParams`, ensuring it must be present in the input data.
- Updated related documentation in `input-sample.md`, `README.md`, and various test scenarios to reflect the change in requirement.
- Adjusted the handling of `date_format` to default to `yyyy-MM-dd HH:mm:ss` if omitted, enhancing usability.
- Refined test scenarios to include new examples and ensure compliance with the updated parameter structure.

These changes improve the robustness of the model training workflow and clarify the expectations for input data.

e2e/test_train_model_validation.py

@@ -6,7 +6,7 @@ ORCHESTRATOR_VALIDATION_ERROR due to invalid parameter values.
 """
 
 import pytest
-import pytest_asyncio
+from temporalio.client import WorkflowFailureError
 from temporalio.testing import WorkflowEnvironment
 from temporalio.worker import Worker
 
@@ -23,6 +23,20 @@ from model_manager.workflows.train_model import TrainModel
 _VALIDATION_ID_BASE = 3000
 
 
+def _exception_chain_text(exc: BaseException) -> str:
+    """Concatenate messages from an exception __cause__/__context__ chain."""
+    parts: list[str] = []
+    cur: BaseException | None = exc
+    seen: set[int] = set()
+    while cur is not None and id(cur) not in seen:
+        seen.add(id(cur))
+        text = str(cur).strip()
+        if text:
+            parts.append(text)
+        cur = cur.__cause__ or getattr(cur, '__context__', None)
+    return ' | '.join(parts).lower()
+
+
 @pytest.mark.asyncio
 @pytest.mark.integration
 async def test_scenario_2_1_1_train_size_out_of_range(
@@ -223,10 +237,12 @@ async def test_scenario_2_1_7_missing_experiment_run_id(
     scenario = load_scenario('01-linear-regression-basic.json')
     scenario = {k: v for k, v in scenario.items() if k != 'experiment_run_id'}
 
-    with pytest.raises(Exception, match='experiment_run_id'):
+    with pytest.raises(WorkflowFailureError) as excinfo:
         await start_and_await_workflow(
             temporal_test_env.client,
             TrainModel.run,
             scenario,
             make_workflow_id('test-s2-1-7'),
         )
+    combined = _exception_chain_text(excinfo.value)
+    assert 'experiment_run_id' in combined