fix: Make sum on strings error in group_by context

[wtn]

Fixes #24659.

Calling sum() on string columns in group_by context silently returned null instead of raising an error.

Reproduction

pl.DataFrame({"str": ["a", "b"]}).group_by(1).agg(pl.col.str.sum())
# Returns null instead of erroring

Expected behavior (as in select context):

pl.DataFrame({"str": ["a", "b"]}).select(pl.col.str.sum())
# InvalidOperationError: `sum` operation not supported for dtype `str`

Cause

String type implements sum_reduce() which correctly raises an error, but was missing an agg_sum() override in the PrivateSeries impl. The default agg_sum() implementation returns null for unsupported types, causing silent failure in group_by aggregations.

Fix

Add agg_sum() override for String type that raises the same error as sum_reduce(), ensuring consistent behavior across select and group_by contexts.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.48%. Comparing base (04e8c78) to head (158b36d).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #25456      +/-   ##
==========================================
+ Coverage   79.39%   79.48%   +0.08%     
==========================================
  Files        1742     1742              
  Lines      240100   240100              
  Branches     3038     3038              
==========================================
+ Hits       190634   190850     +216     
+ Misses      48683    48467     -216     
  Partials      783      783              

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

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[ritchie46]

Thanks for the PR. This should be handled on the IR conversion and raise an InvalidOperation. We shouldn't panic.

[wtn]

Moved the check to AExpr::to_field_impl during schema inference. Raises InvalidOperationError, no panic.