Use Field(default_factory=list) for Page.lines and Page.paragraphs

Pre-existing Optional[List[...]] = [] defaults silently parsed an
explicit JSON null to None, which would cause a TypeError when the
confidence evaluator iterates page.lines (line 132). Switch to the
same Field(default_factory=list) pattern used for the other Page
collections so these fields fail loudly at parse time on a malformed
response and remain safe to iterate at every call site.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: Harsh-Microsoft

src/ContentProcessor/src/libs/azure_helper/model/content_understanding.py

@@ -105,8 +105,8 @@ class Page(BaseModel):
     height: float
     spans: List[Span] = Field(default_factory=list)
     words: List[Word] = Field(default_factory=list)
-    lines: Optional[List[Line]] = []
-    paragraphs: Optional[List[Paragraph]] = []
+    lines: List[Line] = Field(default_factory=list)
+    paragraphs: List[Paragraph] = Field(default_factory=list)
 
 
 class DocumentContent(BaseModel):