refactor(extractor): Convert to stateless pure functions

- Remove DiscreteMaterialPlanExtractor class (stateful)
- Replace with pure functions: chunk_order_ids, get_login_url, extract_batch, etc.
- All functions are stateless, accept explicit parameters
- Caller manages browser/session lifecycle (consistent with extractor_core.py)
- Lower coupling: no direct dependency on utils.auth.login
- Update tests to match new function signatures

Breaking Changes:
- DiscreteMaterialPlanExtractor class removed
- Use extract_and_post_process() or extract_from_file() instead of class methods
- Caller must manage browser session before calling extractor functions

utils/discrete_material_plan/__init__.py

@@ -0,0 +1,43 @@
+"""
+Discrete Material Plan Maintenance Package
+Core web operations and high-level extractor for Yonyou BIP discrete material plan maintenance.
+"""
+
+from .extractor_core import (
+    navigate_to_discrete_material_page,
+    setup_query_interface,
+    fill_and_search_orders,
+    download_batch_data,
+    execute_batch_download_workflow,
+)
+
+from .extractor import (
+    chunk_order_ids,
+    get_login_url,
+    extract_batch,
+    extract_batches,
+    extract_and_post_process,
+    read_order_ids_from_file,
+    extract_from_file,
+)
+
+from .excel_converter import ExcelConverter
+
+__all__ = [
+    # Core functions
+    "navigate_to_discrete_material_page",
+    "setup_query_interface",
+    "fill_and_search_orders",
+    "download_batch_data",
+    "execute_batch_download_workflow",
+    # High-level extractor functions
+    "chunk_order_ids",
+    "get_login_url",
+    "extract_batch",
+    "extract_batches",
+    "extract_and_post_process",
+    "read_order_ids_from_file",
+    "extract_from_file",
+    # Utilities
+    "ExcelConverter",
+]