feat: publish index artifacts through transactions

[yuruiz]

Summary

• publish completed index metadata through Transaction::AddIndexInfo
• persist typed artifact identity, index name, file keys, sizes, row count, and engine versions in manifest Index
• expose the same metadata through LoonIndexInfo and C FFI
• remove ManifestMinorVersion; index presence is represented by manifest indexes
• document the manifest-first, minimal-etcd QueryCoord/QueryNode load design

Testing

• builder-container clang-format dry run
• builder-container build of milvus_test and Test_FFI
• focused manifest / transaction / FFI GTests (5 passed)
• Test_FFI (78 tests, 0 failed)

[sre-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: yuruiz
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[sre-ci-robot]

Welcome @yuruiz! It looks like this is your first PR to milvus-io/milvus-storage 🎉

[codecov]

Codecov Report

❌ Patch coverage is 90.00000% with 20 lines in your changes missing coverage. Please review.
✅ Project coverage is 76.42%. Comparing base (1fd5edf) to head (5bafce4).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
cpp/src/ffi/bridge.cpp	86.40%	14 Missing ⚠️
cpp/src/ffi/manifest_c.cpp	90.16%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #620      +/-   ##
==========================================
+ Coverage   76.19%   76.42%   +0.23%     
==========================================
  Files         173      173              
  Lines       17388    17561     +173     
  Branches     2618     2655      +37     
==========================================
+ Hits        13248    13421     +173     
  Misses       4140     4140              

Flag	Coverage Δ
cpp	79.01% <90.00%> (+0.22%)	⬆️
python	44.45% <ø> (ø)

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[yuruiz]

Addressed the latest review comments in commit 5bafce4:

• kept Index::properties as std::unordered_map for the common key-lookup path
• split legacy raw MILV index decoding by manifest version; v2-v5 use the four-field layout, while v6+ uses the typed layout
• added compatibility comments and kept LoonIndexInfo limited to completed artifact metadata; IsDeleted/IndexState remain transient build-task state

Validation: legacy v2/v3 tests passed and Test_FFI passed 78/78.

[jiaqizho]

/lgtm