refactor(plan_node): simplify union nodes

[ice1000]

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What's changed and what's your intention?

Follow up of #8898, #8870, #8905, #8930, #8932, #9011, #9044, #9176, #9715, #9740, #9785.

This is also useful in the series of #10157

Checklist For Contributors

• Yes

Documentation

• My PR DOES NOT contain user-facing changes.

[codecov]

Codecov Report

Merging #10198 (728ed50) into main (572780b) will decrease coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main   #10198      +/-   ##
==========================================
- Coverage   70.79%   70.79%   -0.01%     
==========================================
  Files        1235     1235              
  Lines      211451   211447       -4     
==========================================
- Hits       149698   149686      -12     
- Misses      61753    61761       +8     

Flag	Coverage Δ
rust	70.79% <100.00%> (-0.01%)	⬇️

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

Impacted Files	Coverage Δ
...rc/frontend/src/optimizer/plan_node/batch_union.rs	89.58% <100.00%> (-0.22%)	⬇️
...rc/frontend/src/optimizer/plan_node/logical_agg.rs	97.04% <100.00%> (ø)
.../frontend/src/optimizer/plan_node/logical_union.rs	97.34% <100.00%> (-2.26%)	⬇️
...c/frontend/src/optimizer/plan_node/stream_union.rs	98.03% <100.00%> (-0.43%)	⬇️

... and 2 files with indirect coverage changes

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[st1page]

LGTM