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.