Refactor Etcdserver.snapshot to extract compaction into a separate method

[ahrtr]

Since nobody proceeds with the proposal, so create a followup PR to https://github.com/etcd-io/etcd/pull/18635/files#r1778755879

cc @clement2026 @serathius

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 68.75%. Comparing base (36d785d) to head (976e9ee).
Report is 12 commits behind head on main.

❗ Current head 976e9ee differs from pull request most recent head 694b0fb

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Additional details and impacted files

Files with missing lines	Coverage Δ
server/etcdserver/server.go	81.32% <100.00%> (-0.11%)	⬇️

... and 19 files with indirect coverage changes

@@            Coverage Diff             @@
##             main   #18808      +/-   ##
==========================================
- Coverage   68.81%   68.75%   -0.07%     
==========================================
  Files         420      420              
  Lines       35512    35515       +3     
==========================================
- Hits        24437    24417      -20     
- Misses       9647     9669      +22     
- Partials     1428     1429       +1     

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