Docs say RouteMatrixElement.duration is Duration, but runtime returns datetime.timedelta

### Determine this is the right repository

- [x] I determined this is the correct repository in which to report this bug.

### Summary of the issue

**Context**
I was using `google-maps-routing` and inspecting `RouteMatrixElement.duration` from a `compute_route_matrix` response.

**Expected Behavior:**
The documentation indicates that `RouteMatrixElement.duration` is a `google.protobuf.duration_pb2.Duration`.

**Actual Behavior:**
At runtime, `RouteMatrixElement.duration` is returned as a `datetime.timedelta` object.

This appears to be a documentation or generated type-surface mismatch rather than a server-side API problem. The underlying protobuf field may be `Duration`, but the Python client returns the marshaled native Python type.

### API client name and version

google-maps-routing 0.10.0

### Reproduction steps: code

file: main.py
```python
from datetime import timedelta
from google.maps import routing_v2

def main():
client = routing_v2.RoutesClient()

request = routing_v2.ComputeRouteMatrixRequest(
origins=[
routing_v2.RouteMatrixOrigin(
waypoint=routing_v2.Waypoint(
address="1600 Amphitheatre Parkway, Mountain View, CA"
)
)
],
destinations=[
routing_v2.RouteMatrixDestination(
waypoint=routing_v2.Waypoint(
address="1 Infinite Loop, Cupertino, CA"
)
)
],
travel_mode=routing_v2.RouteTravelMode.DRIVE,
)

stream = client.compute_route_matrix(
request=request,
metadata=[(
"x-goog-fieldmask",
"originIndex,destinationIndex,status,condition,distanceMeters,duration"
)],
)

element = next(iter(stream))

print("type:", type(element.duration))
print("is timedelta:", isinstance(element.duration, timedelta))

if __name__ == "__main__":
main()
```

### Reproduction steps: supporting files

### Reproduction steps: actual results

```
type:
is timedelta: True
```

### Reproduction steps: expected results

```
type:
is timedelta: False
```

### OS & version + platform

Window 11 25H2

### Python environment

Python 3.11.9

### Python dependencies

pip list
| Package | Version |
|---------------------------|--------------|
| altgraph | 0.17.5 |
| certifi | 2026.2.25 |
| cffi | 2.0.0 |
| charset-normalizer | 3.4.6 |
| cryptography | 46.0.6 |
| google-api-core | 2.30.1 |
| google-auth | 2.49.1 |
| google-geo-type | 0.6.0 |
| google-maps-routing | 0.10.0 |
| googleapis-common-protos | 1.73.1 |
| greenlet | 3.3.2 |
| grpcio | 1.80.0 |
| grpcio-status | 1.80.0 |
| idna | 3.11 |
| jellyfish | 1.2.1 |
| packaging | 26.0 |
| pefile | 2024.8.26 |
| pip | 24.2 |
| proto-plus | 1.27.2 |
| protobuf | 6.33.6 |
| pyasn1 | 0.6.3 |
| pyasn1_modules | 0.4.2 |
| pycparser | 3.0 |
| pyinstaller | 6.19.0 |
| pyinstaller-hooks-contrib | 2026.4 |
| pyodbc | 5.3.0 |
| python-dateutil | 2.9.0.post0 |
| python-dotenv | 1.2.2 |
| pywin32-ctypes | 0.2.3 |
| requests | 2.33.1 |
| setuptools | 82.0.1 |
| six | 1.17.0 |
| SQLAlchemy | 2.0.48 |
| typing_extensions | 4.15.0 |
| urllib3 | 2.6.3 |
| wheel | 0.44.0 |

### Additional context

This appears to be a documentation or generated type-surface mismatch rather than a server-side Routes API bug.

The docs for `RouteMatrixElement.duration` describe the protobuf field type (`google.protobuf.duration_pb2.Duration`), but at runtime the Python client returns the marshaled native Python type (`datetime.timedelta`). That makes the docs misleading when trying to write type-aware Python code.

If this behavior is intentional because of proto-plus marshalling, the documentation should reflect the runtime Python type. If it is not intentional, then the runtime type is inconsistent with the documented type.

Commencez par la reproduction dans main.py et examinez la surface de types documentée et générée pour routing_v2.RouteMatrixElement.duration, en la comparant à la valeur datetime.timedelta renvoyée par compute_route_matrix. Confirmez si le comportement du client est intentionnel, puis mettez à jour la documentation ou la description de type concernée afin que les types Python indiqués et observés correspondent.

Rédigé par le modèle d'indexation à partir du texte de l'issue.