explain(streaming=True) isn't showing correct plan

[Thomzoy]

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 polars as pl
pl.Config.set_streaming_chunk_size(2)

a = pl.LazyFrame(
            data=dict(
                a=[1,2,3,3],
            )
        )
b = pl.LazyFrame(
            data=dict(
                a=[1,2,3,3],
            )
        )

joined = a.join(b, on="a")
joined .explain(streaming=True)

Log output

STREAMING
  DF []; PROJECT */0 COLUMNS; SELECTION: None

Issue description

Description

When running a simple join on LazyFrames in a streaming fashion, the plan printed by explain seems to be incorrect

Example

import polars as pl
pl.Config.set_streaming_chunk_size(2)

a = pl.LazyFrame(
            data=dict(
                a=[1,2,3,3],
            )
        )
b = pl.LazyFrame(
            data=dict(
                a=[1,2,3,3],
            )
        )

joined = a.join(b, on="a")

joined .explain(streaming=False)

INNER JOIN:
LEFT PLAN ON: [col("a")]
  DF ["a"]; PROJECT */1 COLUMNS; SELECTION: None
RIGHT PLAN ON: [col("a")]
  DF ["a"]; PROJECT */1 COLUMNS; SELECTION: None
END INNER JOIN

joined .explain(streaming=True)

STREAMING
  DF []; PROJECT */0 COLUMNS; SELECTION: None

joined.collect(streaming=True)

However, collecting with streaming seems to display the correct streaming pipeline:

RUN STREAMING PIPELINE
[df -> callback -> ordered_sink, df -> generic_join_build]

Expected behavior

Display the correct plan

Installed versions

--------Version info---------
Polars:               0.20.31
Index type:           UInt32
Platform:             Linux-4.18.0-425.10.1.el8_7.x86_64-x86_64-with-glibc2.35
Python:               3.11.9 (main, Apr 19 2024, 16:48:06) [GCC 11.2.0]

----Optional dependencies----
adbc_driver_manager:  <not installed>
cloudpickle:          <not installed>
connectorx:           <not installed>
deltalake:            0.17.4
fastexcel:            <not installed>
fsspec:               2024.5.0
gevent:               <not installed>
hvplot:               <not installed>
matplotlib:           <not installed>
nest_asyncio:         1.6.0
numpy:                1.26.4
openpyxl:             <not installed>
pandas:               2.2.2
pyarrow:              16.1.0
pydantic:             <not installed>
pyiceberg:            <not installed>
pyxlsb:               <not installed>
sqlalchemy:           <not installed>
torch:                <not installed>
xlsx2csv:             <not installed>
xlsxwriter:           <not installed>