[Python] Keep the Series name in DataFrame Series.agg with a list

[anishmehta24]

Fixes #40572

Hand-implemented aggregations (mean, std, var, ...) inside a list were wrapped with pd.Series(x, index=[base_func]) without a name, so s.agg(['sum', 'mean']) lost the Series name. The wrapper now passes the name from the proxy. Added test_series_agg_list_keeps_name (fails on master); frames_test.py and pandas_doctests_test.py pass.

Written with help from Claude.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.94%. Comparing base (76f6e6d) to head (cafc44d).
⚠️ Report is 160 commits behind head on master.

Additional details and impacted files

@@              Coverage Diff              @@
##             master   #40573       +/-   ##
=============================================
+ Coverage     56.16%   79.94%   +23.77%     
=============================================
  Files          1124      488      -636     
  Lines        178336    82348    -95988     
  Branches       1489        0     -1489     
=============================================
- Hits         100170    65830    -34340     
+ Misses        75645    16518    -59127     
+ Partials       2521        0     -2521     

Flag	Coverage Δ
go	?
java	?
python	79.94% <100.00%> (+0.38%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Harness.
📢 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.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[github-actions]

Assigning reviewers:

R: @claudevdm for label python.
Backup reviewer: jrmccluskey (if the main reviewer is unresponsive, you can escalate to them by tagging their username)

Note: If you would like to opt out of this review, comment assign to next reviewer.

Available commands:

• stop reviewer notifications - opt out of the automated review tooling
• remind me after tests pass - tag the comment author after tests pass
• waiting on author - shift the attention set back to the author (any comment or push by the author will return the attention set to the reviewers)

The PR bot will only process comments in the main thread (not review comments).