CHORE: Fix PerformanceWarning in pivot combine stage

[ChengjieLi28]

What do these changes do?

Fix this warning in pivot op combine stage.

performancewarning: dataframe is highly fragmented. this is usually the result of calling `frame.insert` many times, which has poor performance. consider joining all columns at once using pd.concat(axis=1) instead. to get a de-fragmented frame, use `newframe = frame.copy()`

Related issue number

Fixes #xxxx

Check code requirements

• tests added / passed (if needed)
• Ensure all linting tests pass

[codecov]

Codecov Report

❌ Patch coverage is 0% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 49.62%. Comparing base (b320ca3) to head (f1587de).
⚠️ Report is 63 commits behind head on main.

Files with missing lines	Patch %	Lines
python/xorbits/_mars/dataframe/base/pivot.py	0.00%	3 Missing ⚠️

❌ Your project check has failed because the head coverage (49.62%) is below the target coverage (90.00%). You can increase the head coverage or adjust the target coverage.

❗ There is a different number of reports uploaded between BASE (b320ca3) and HEAD (f1587de). Click for more details.

HEAD has 20 uploads less than BASE

Flag	BASE (b320ca3)	HEAD (f1587de)
unittests	22	2

Additional details and impacted files

@@             Coverage Diff             @@
##             main     #747       +/-   ##
===========================================
- Coverage   82.55%   49.62%   -32.93%     
===========================================
  Files        1058     1058               
  Lines       79780    79779        -1     
  Branches    16504    12433     -4071     
===========================================
- Hits        65861    39593    -26268     
- Misses      11645    38595    +26950     
+ Partials     2274     1591      -683     

Flag	Coverage Δ
unittests	49.61% <0.00%> (-32.81%)	⬇️

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.