community: Include PDF ID in MathPix metadata (#15629)

- **Description:** Includes the PDF ID in the MathPix document metadata.
This is useful in case you need to re-request a processed PDF from the
MathPix API later.

libs/community/langchain_community/document_loaders/pdf.py

@@ -518,7 +518,7 @@ def load(self) -> List[Document]:
         contents = self.get_processed_pdf(pdf_id)
         if self.should_clean_pdf:
             contents = self.clean_pdf(contents)
-        metadata = {"source": self.source, "file_path": self.source}
+        metadata = {"source": self.source, "file_path": self.source, "pdf_id": pdf_id}
         return [Document(page_content=contents, metadata=metadata)]