Add nuraft_state field for complete Raft state persistence

[Besroy]

This change use NuRaft's native binary serialization to persist srv_state instead of manual JSON field mapping. This ensures all state fields including receiving_snapshot_ are saved, preventing state inconsistency and unexpected raft behavior.

related issue: #835

[codecov-commenter]

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Codecov Report

❌ Patch coverage is 7.14286% with 13 lines in your changes missing coverage. Please review.
✅ Project coverage is 49.54%. Comparing base (1a0cef8) to head (045681b).
⚠️ Report is 299 commits behind head on master.

Files with missing lines	Patch %	Lines
src/lib/replication/repl_dev/raft_repl_dev.cpp	7.14%	1 Missing and 12 partials ⚠️
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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #841      +/-   ##
==========================================
- Coverage   56.51%   49.54%   -6.97%     
==========================================
  Files         108      110       +2     
  Lines       10300    11329    +1029     
  Branches     1402     5345    +3943     
==========================================
- Hits         5821     5613     -208     
+ Misses       3894     2080    -1814     
- Partials      585     3636    +3051     

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[yuwmao]

LGTM