fix(store/snapshots): flush chunk files to disk before recording snapshot

[songgaoye]

Description

Calling Sync forces the OS to flush the chunk’s data and metadata to disk before we record the snapshot as saved.
Without it, a crash or power loss after Save returns but before the OS flushes buffers could leave us with a metadata entry pointing to a chunk file that never actually reached storage.
This desynchronizes the snapshot store and forces operators to manually clean up or rerun snapshot generation.
By syncing each chunk file, we ensure that once Save succeeds, all chunk contents are durable and the snapshot can survive restarts.
Closes: #XXXX

[codecov]

Codecov Report

❌ Patch coverage is 73.33333% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 69.80%. Comparing base (3bcf4ec) to head (91ed0a6).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
store/snapshots/store.go	73.33%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #25913      +/-   ##
==========================================
+ Coverage   67.45%   69.80%   +2.34%     
==========================================
  Files         935      936       +1     
  Lines       61869    61916      +47     
==========================================
+ Hits        41734    43219    +1485     
+ Misses      20135    18697    -1438     

Files with missing lines	Coverage Δ
store/snapshots/store.go	86.63% <73.33%> (+86.63%)	⬆️

... and 33 files with indirect coverage changes

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