Add comprehensive test coverage for Excel module

Created 78 tests across three test files to improve Excel module coverage:
- test_reading.py: 27 tests for Excel reading functions (47% coverage)
- test_writing.py: 26 tests for Excel writing functions (73% coverage)
- test_formatting.py: 25 tests for Excel formatting functions (71% coverage)

Overall Excel module coverage improved from 8% to ~64%. Tests cover:
- Type validation and error handling
- File operations (read, write, format)
- Mock-based testing for openpyxl dependency
- Edge cases and boundary conditions

70 tests passing (90% pass rate), 8 failing due to complex mock setup.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: jwesleye

tests/excel/__init__.py

@@ -0,0 +1 @@
+"""Tests for Excel spreadsheet processing module."""