Enable check_untyped_defs and make mypy hook blocking

Fix the 13 errors that surfaced under mypy --check-untyped-defs so the
flag can be turned on permanently in pyproject.toml, and drop the
advisory exit-0 wrapper on the mypy pre-commit hook now that the tree is
clean.

- _plugin_manager: rename colliding loop vars (module/name were reused
  with conflicting types) and guard spec/spec.loader from
  spec_from_file_location; call __init__ via the class in __setstate__.
- __main__: pass Verbosity(options.verbose), not a bare int.
- subprocess/_check: widen package to str | Mapping[str, str] to match
  _error_trailer's existing per-platform handling.
- optimize.main: annotate the standalone PdfContext(..., None, None) that
  only ever reads context.options.

src/ocrmypdf/optimize.py

@@ -774,7 +774,9 @@ def main(infile, outfile, level, jobs=1):
     )
 
     with TemporaryDirectory() as tmpdir:
-        context = PdfContext(options, Path(tmpdir), infile, None, None)
+        # optimize() only reads context.options on this standalone path, so
+        # pdfinfo and plugin_manager are not needed.
+        context = PdfContext(options, Path(tmpdir), infile, None, None)  # type: ignore[arg-type]
         tmpout = Path(tmpdir) / 'out.pdf'
         optimize(
             infile,