Include docstring of field definitions in LM prompt if applicable.

Usage:
```python
class Step(pg.Object):
  description: str
  output: pg.typing.Annotated[
    float,
    'Output for current step based on the steps of previous steps.'
  ]

class Solution(pg.Object):
  steps: list[Step]
  final_answer: int

lf.query(question, Solution)
```

PiperOrigin-RevId: 569547439

langfun/core/structured/schema.py

@@ -360,7 +360,11 @@ def class_definition(cls, strict: bool = False) -> str:
             'Variable-length keyword arguments is not supported in '
             f'structured parsing or query. Encountered: {field}'
         )
-      out.write(f'  {field.key}: {annotation(field.value, strict=strict)}\n')
+      out.write(f'  {field.key}: {annotation(field.value, strict=strict)}')
+      if field.description:
+        description = field.description.replace('\n', ' ')
+        out.write(f'  # {description}')
+      out.write('\n')
   else:
     out.write('  pass\n')
   return out.getvalue()

langfun/core/structured/schema_test.py

@@ -29,7 +29,10 @@ class Itinerary(pg.Object):
   day: pg.typing.Int[1, None]
   type: pg.typing.Enum['daytime', 'nighttime']
   activities: list[Activity]
-  hotel: pg.typing.Str['.*Hotel'] | None
+  hotel: pg.typing.Annotated[
+      pg.typing.Str['.*Hotel'] | None,
+      'Hotel to stay if applicable.'
+  ]
 
 
 class Node(pg.Object):
@@ -387,7 +390,7 @@ class Itinerary:
               day: int(min=1)
               type: Literal['daytime', 'nighttime']
               activities: list[Activity]
-              hotel: str(regex='.*Hotel') | None
+              hotel: str(regex='.*Hotel') | None  # Hotel to stay if applicable.
             """) + '\n',
     )