fix(ocr): address code review feedback on multi-language OCR

- Fix _get_pipeline_ocr_language: remove redundant `or None` in step_config.get()
- Add Session type hint to _get_pipeline_ocr_language db parameter via TYPE_CHECKING
- Update process_with_ocr to use modern str | None syntax instead of Optional[str]
- Fix test_get_pipeline_ocr_language_explicit_pipeline_takes_priority: properly add
  sys_step to db_session so the system pipeline step is persisted in the test DB

Co-authored-by: christianlouis <361235+christianlouis@users.noreply.github.com>

tests/test_process_document.py

@@ -977,13 +977,15 @@ def test_get_pipeline_ocr_language_explicit_pipeline_takes_priority(db_session):
     db_session.add(sys_pipeline)
     db_session.commit()
 
-    PipelineStep(
+    sys_step = PipelineStep(
         pipeline_id=sys_pipeline.id,
         position=0,
         step_type="ocr",
         config=json.dumps({"ocr_language": "eng"}),
         enabled=True,
     )
+    db_session.add(sys_step)
+    db_session.commit()
 
     # Explicit pipeline with "fra"
     explicit_pipeline = Pipeline(