refactor(logging): Add optional logging support throughout codebase

Add centralized logging utility and optional logger parameters to all
core functions for better observability and debugging capabilities.

New modules:
- utils/logging.py: Centralized logger configuration with console
  and optional file handlers

Enhanced features:
- Added optional logger parameter to all extractor_core functions
- Added logger support to extractor, excel_converter, and auth modules
- Functions remain silent when logger=None (backward compatible)
- Improved environment variable validation in test files

Documentation:
- Added discrete_material_plan_extractor_core.md with complete API
  reference and usage patterns

Benefits:
- Consistent logging format across all components
- Optional debug output for troubleshooting
- No breaking changes - fully backward compatible
- Better error messages and validation

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tests/test_auth_config.py

@@ -12,7 +12,10 @@ PROJECT_ROOT = Path(__file__).resolve().parent.parent
 load_dotenv(PROJECT_ROOT / ".env")
 
 # Configure browser path
-os.environ["PLAYWRIGHT_BROWSERS_PATH"] = os.getenv("PLAYWRIGHT_BROWSERS_PATH")
+browser_path = os.getenv("PLAYWRIGHT_BROWSERS_PATH")
+if not browser_path:
+    raise ValueError("PLAYWRIGHT_BROWSERS_PATH environment variable is required")
+os.environ["PLAYWRIGHT_BROWSERS_PATH"] = browser_path
 
 print("=" * 50)
 print("ERP System Configuration Check")