docs: Add example and tests for pl.concat() with Expr input

[etiennebacher]

Fixes #19813

[codecov]

Codecov Report

Attention: Patch coverage is 85.34075% with 527 lines in your changes missing coverage. Please review.

Project coverage is 79.50%. Comparing base (6ccb187) to head (1328485).
Report is 69 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/polars-row/src/encode.rs	81.12%	124 Missing ⚠️
...tream/src/nodes/parquet_source/row_group_decode.rs	0.00%	75 Missing ⚠️
...rates/polars-compute/src/horizontal_flatten/mod.rs	60.28%	56 Missing ⚠️
...lars-core/src/series/arithmetic/fixed_size_list.rs	92.49%	41 Missing ⚠️
crates/polars-ops/src/series/ops/round.rs	63.20%	39 Missing ⚠️
crates/polars-row/src/decode.rs	85.96%	24 Missing ⚠️
crates/polars-row/src/variable.rs	85.48%	18 Missing ⚠️
crates/polars-schema/src/schema.rs	55.88%	15 Missing ⚠️
.../polars-ops/src/chunked_array/strings/find_many.rs	73.07%	14 Missing ⚠️
crates/polars-row/src/row.rs	67.74%	10 Missing ⚠️
... and 32 more

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #19836      +/-   ##
==========================================
+ Coverage   79.33%   79.50%   +0.16%     
==========================================
  Files        1548     1556       +8     
  Lines      214245   216530    +2285     
  Branches     2460     2457       -3     
==========================================
+ Hits       169968   172145    +2177     
- Misses      43719    43827     +108     
  Partials      558      558              

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[eitsupi]

I think vertical should be checked for Expr as well.

[ritchie46]

Suggested change

	This is not used when items are Expr. Series only support the `vertical` strategy.
	Series and Expr only support the `vertical` strategy.

[etiennebacher]

Test failures only appear with the new streaming engine but I have no idea why. Should I remove the new test?

[eitsupi]

I suppose you can skip tests with @pytest.mark.may_fail_auto_streaming for now.

[etiennebacher]

Thanks, it passes now

[etiennebacher]

Ran into issues when rebasing trying to rebase, will open a new one

[eitsupi]

If you're new to Git, I recommend that you don't try to rebase.

[etiennebacher]

Not new, but I never use that (and when I do I rely on the github interface). I see it's the norm here, which is why I tried

[eitsupi]

There would have been no need to close the PR if the branch had been reset to the state it was in before the rebase attempt was made.