When doing group_by_dynamic, cannot access "_upper_boundary" in map_groups function.

[lzs-raja]

Checks

• I have checked that this issue has not already been reported.
• I have confirmed this bug exists on the latest version of Polars.

Reproducible example

import datetime
df = pl.DataFrame(
    {
        "x": [
            datetime.datetime(2023, 1, 1, 0, 0, 0),
            datetime.datetime(2023, 1, 1, 0, 1, 0),
            datetime.datetime(2023, 1, 1, 0, 5, 0),
            datetime.datetime(2023, 1, 1, 0, 10, 0),
            datetime.datetime(2023, 1, 1, 0, 11, 0),
        ],
        "y": [1, 2, 3, 4, 5],
    }
).sort("x")
def test_map(group):
    time = group["_upper_boundary"][-1] 
    x = group["x"][-1]
    return pl.DataFrame({"time":[time],"x":x})
agg_df = df.group_by_dynamic(index_column="x", every="5m", period="10m", offset="5m", include_boundaries=True).map_groups(test_map, schema=None)
print(agg_df)

Log output

ComputeError: User provided python function failed: ColumnNotFoundError: "_upper_boundary" not found

Issue description

When doing group_by_dynamic, "agg" can have _upper_boundary if set include_boundaries=True, but "map_group" cannot access this. Though i can't access this, at least add _upper_boundary to the result like what is done in "agg". But the fact is not.

Expected behavior

Either add _upper_boundary to the result like "agg" result, or I can access group["_upper_boundary"][-1] or group._upper_boundary and use them in the function result

Installed versions

--------Version info---------
Polars:              1.19.0
Index type:          UInt32
Platform:            Windows-10-10.0.22621-SP0
Python:              3.9.19 (main, May  6 2024, 20:12:36) [MSC v.1916 64 bit (AMD64)]
LTS CPU:             False

----Optional dependencies----
adbc_driver_manager  
<not installed>
altair               <not installed>
azure.identity       <not installed>
boto3                <not installed>
cloudpickle          <not installed>
connectorx           <not installed>
deltalake            <not installed>
fastexcel            <not installed>
fsspec               <not installed>
gevent               <not installed>
google.auth          <not installed>
great_tables         <not installed>
matplotlib           3.8.4
nest_asyncio         1.6.0
numpy                1.24.3
openpyxl             <not installed>
pandas               2.2.1
pyarrow              <not installed>
pydantic             <not installed>
pyiceberg            <not installed>
sqlalchemy           <not installed>
torch                2.3.0
xlsx2csv             <not installed>
xlsxwriter           <not installed>```

</details>