refactor(plan_node): simplify batch agg-nodes

[ice1000]

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What's changed and what's your intention?

Follow up of #8898, #8870, #8905, #8930, #8932, #9011, #9044.

Checklist For Contributors

• Yes

Documentation

• My PR DOES NOT contain user-facing changes.

[codecov]

Codecov Report

Merging #9176 (678ee1f) into main (fdd319e) will increase coverage by 0.00%.
The diff coverage is 96.82%.

@@           Coverage Diff           @@
##             main    #9176   +/-   ##
=======================================
  Coverage   70.83%   70.84%           
=======================================
  Files        1200     1200           
  Lines      200057   200043   -14     
=======================================
- Hits       141720   141714    -6     
+ Misses      58337    58329    -8     

Flag	Coverage Δ
rust	70.84% <96.82%> (+<0.01%)	⬆️

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

Impacted Files	Coverage Δ
...frontend/src/optimizer/plan_node/batch_sort_agg.rs	78.16% <93.75%> (-0.50%)	⬇️
...frontend/src/optimizer/plan_node/batch_hash_agg.rs	80.00% <94.11%> (-0.38%)	⬇️
...rc/frontend/src/optimizer/plan_node/generic/agg.rs	84.28% <100.00%> (+0.45%)	⬆️
...rc/frontend/src/optimizer/plan_node/logical_agg.rs	96.28% <100.00%> (-0.34%)	⬇️
.../frontend/src/optimizer/plan_node/logical_union.rs	99.60% <100.00%> (ø)

... and 5 files with indirect coverage changes

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