Fix descriptions being dropped in some places for protein/RNA chains

* From #677 by @ntnn19. Thank you!

PiperOrigin-RevId: 919589563
Change-Id: I82c6ccf0797b87d9347fda731af00b9c0cc16cea

Author: Augustin-Zidek

src/alphafold3/common/folding_input.py

@@ -648,6 +648,7 @@ def fill_missing_fields(self) -> Self:
         id=self.id,
         sequence=self.sequence,
         modifications=self.modifications,
+        description=self.description,
         unpaired_msa=self._unpaired_msa or '',
     )
 

src/alphafold3/data/pipeline.py

@@ -538,6 +538,7 @@ def process_protein_chain(
         id=chain.id,
         sequence=chain.sequence,
         ptms=chain.ptms,
+        description=chain.description,
         unpaired_msa=unpaired_msa,
         paired_msa=paired_msa,
         templates=templates,
@@ -570,6 +571,7 @@ def process_rna_chain(
         id=chain.id,
         sequence=chain.sequence,
         modifications=chain.modifications,
+        description=chain.description,
         unpaired_msa=unpaired_msa,
     )