Fix hnsw index could not be mmapped after restart

[euvre]

What problem does this PR solve?

Because the hnsw buffer_obj disappears after a restart, when it is reconstructed its state is set to kPersistent instead of kMmap. On the relevant code paths you should manually set it to kMmap before reading the index.

Type of change

• Bug Fix (non-breaking change which fixes an issue)

[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 42.79%. Comparing base (0c5df9d) to head (7fe396f).
⚠️ Report is 5 commits behind head on main.

Files with missing lines	Patch %	Lines
src/storage/buffer/buffer_obj_impl.cpp	0.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3274      +/-   ##
==========================================
- Coverage   50.54%   42.79%   -7.75%     
==========================================
  Files         731      731              
  Lines      153402   153403       +1     
  Branches    27752    27752              
==========================================
- Hits        77534    65648   -11886     
- Misses      65532    78193   +12661     
+ Partials    10336     9562     -774     

Flag	Coverage Δ
debug http test	28.15% <50.00%> (+0.08%)	⬆️
debug parallel test	17.45% <50.00%> (?)
debug pysdk test	?
debug sqllogical test	?
debug unit test	38.63% <50.00%> (-0.01%)	⬇️

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