This fixes regression in headless mode test that caused Windows CI/CD to
fail with IndentationError. It replaces python -c multi-line string with
heredoc to eliminate YAML indentation issues.

Problem:
- YAML run: | preserves leading whitespace from indentation
- Multi-line Python code in -c had leading spaces on each line
- Python interpreter rejected code with unexpected indent
- Error: "Process completed with exit code 1" on Windows

Root cause:
  python -c "
  from kconfiglib import Kconfig    # ← Leading spaces from YAML
  import menuconfig                  # ← Causes IndentationError
  ...
  "

Solution:
- Use heredoc (python << 'EOF') instead of python -c
- Heredoc is the standard bash method for multi-line input
- Python receives clean code from stdin without leading spaces
- Maintains code readability with proper formatting

  python << 'EOF'
  from kconfiglib import Kconfig
  import menuconfig
  ...
  EOF