python: primitive oneOf (after deserialization work-around) does not round-trip/reserialize

### What are you generating using Kiota, clients or plugins?

API Client/SDK

### In what context or format are you using Kiota?

Nuget tool

### Client library/SDK language

Python

### Describe the bug

After making the fix to read a primitive oneOf (#6868 ) the scalar value is lost (set to `{}`) when re-serialized.

### Expected behavior

scalar value should be re-seralized

### How to reproduce

```
obj = <...call my kiota API with scalar oneOf value...>
writer = JsonSerializationWriter()
writer.write_object_value(None, obj)
# We've lost scalar_value prop at this point (other props ok)
bytes = writer.get_serialized_content()
json = bytes.decode('utf-8')
```
So we get to this part:
```
def serialize(self,writer: SerializationWriter) -> None:
....
writer.write_object_value("scalarValue", self.scalar_value)
```

This gets executed correctly:
```
def serialize(self,writer: SerializationWriter) -> None:
...
elif self.double:
writer.write_float_value(None, self.double) # <== Correct
```
but then we get here:
```
def write_object_value(
self, key: Optional[str], value: Optional[U], *additional_values_to_merge: Optional[U]
) -> None:
...
if value or additional_values_to_merge:
temp_writer = self._create_new_writer()
...
if value:
self._serialize_value(temp_writer, value)
...
if key:
# key = 'scalarValue',
# temp_writer.value = 42.0
# However, temp_write.writer is {}, so the value we set is lost
self.writer[key] = temp_writer.writer
else:
self.value = temp_writer.writer
```
So when we finally exit `serialize()`, all the other fields have been set on the object, but the scalar value is not `42.0` but `{}`.

### Open API description file

_No response_

### Kiota Version

1.9.6

### Latest Kiota version known to work for scenario above?(Not required)

_No response_

### Known Workarounds

_No response_

### Configuration

Win x64

### Debug output

Click to expand log
```

```

### Other information

_No response_

Start with JsonSerializationWriter.write_object_value and the scalar oneOf serialize path shown in the report. Reproduce the issue using the provided snippet and inspect how the temporary writer handles the scalar value. Done means a primitive oneOf value such as 42.0 survives serialization and is present in the resulting JSON.

Written by the indexing model from the issue text.