Fix Windows regression: use heredoc instead

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

Author: jserv

.github/workflows/test.yml

@@ -92,15 +92,14 @@ jobs:
       # Use root dir for Windows, Kconfiglib subdir for Linux/macOS
       working-directory: ${{ matrix.target.headless-only && '.' || 'Kconfiglib' }}
       run: |
-        python -c "
+        python << 'EOF'
         from kconfiglib import Kconfig
         import menuconfig
-
         print('Testing headless mode...')
         kconf = Kconfig('examples/Kmenuconfig')
         menuconfig.menuconfig(kconf, headless=True)
-        print('✅ Headless mode test passed')
-        "
+        print('Headless mode test passed')
+        EOF
 
     - name: Install windows-curses (Windows only)
       if: matrix.target.os == 'Windows'
@@ -112,4 +111,4 @@ jobs:
       # Use root dir for Windows (headless-only mode)
       working-directory: ${{ matrix.target.headless-only && '.' || 'Kconfiglib' }}
       run: |
-        python -c "import menuconfig; print('✅ menuconfig import successful')"
+        python -c "import menuconfig; print('menuconfig import successful')"