Add 'auto' output type for best-effort PDF/A without Ghostscript

- Add new '--output-type auto' option (now the default) that produces
  best-effort PDF/A without requiring Ghostscript
- When verapdf is available, use speculative PDF/A conversion
- Without verapdf, pass through as PDF/A if safe (input claims PDF/A
  or --force-ocr was used), otherwise output as regular PDF
- Make Ghostscript check conditional - only required for pdfa* output types
- Update soft error tests to explicitly use --output-type pdfa since they
  exercise Ghostscript failure modes
- Fix Tesseract OSD error handling to check both stdout and stderr for
  known non-fatal messages like "Too few characters"

tests/test_soft_error.py

@@ -41,6 +41,8 @@ def test_render_continue_on_soft_error(resources, outpdf):
     exitcode = run_ocrmypdf_api(
         resources / 'francais.pdf',
         outpdf,
+        '--output-type',
+        'pdfa',  # Required to trigger Ghostscript PDF/A generation
         '--continue-on-soft-render-error',
         '--plugin',
         'tests/plugins/tesseract_noop.py',
@@ -55,6 +57,8 @@ def test_render_stop_on_soft_error(resources, outpdf):
     exitcode = run_ocrmypdf_api(
         resources / 'francais.pdf',
         outpdf,
+        '--output-type',
+        'pdfa',  # Required to trigger Ghostscript PDF/A generation
         '--plugin',
         'tests/plugins/tesseract_noop.py',
         '--plugin',