feat: add IndexParam for index creation and deprecate IndexDesc-based request input

[yhmo]

What this PR does / why we need it

Introduce a dedicated IndexParam type for index-creation input so the C++ SDK matches the Java/Rust SDKs, where createIndex takes IndexParam and describeIndex returns IndexDesc. Previously the C++ SDK reused IndexDesc (the describe type) as the create-index input.

What changes

• New public type milvus::IndexParam (src/include/milvus/types/IndexParam.h): creation-only fields field_name/index_name/index_type/metric_type/extra_params.
• CreateIndexRequest and CreateCollectionRequest (inline index creation) gain IndexParams() / SetIndexParams() / WithIndexParams() / AddIndexParam() to build multiple indexes from IndexParam.
• The previous IndexDesc-based accessors (Indexes() / SetIndexes() / WithIndexes() / AddIndex()) are marked [[deprecated]] but remain functional: deprecated setters forward into the IndexParam list, and the deprecated Indexes() getter derives the IndexDesc view on demand, so existing callers keep working without a source break (single source of truth).
• MilvusClientV2::CreateIndex / CreateCollection signatures are unchanged; the impl builds the create-index RPC from IndexParam entries.
• Examples, tutorials, and unit/mocked/system tests updated to the new accessors.

Verification

• testing-ut: 923 tests passed
• testing-it (mocked): CreateIndex/CreateCollection suites passed
• testing-st (system tests) compiles
• All 40 V2 examples build
• All 6 tutorials compile against the PR SDK
• All changed files pass clang-format

Notes

• AddFunctionFieldRequest::WithIndex still uses IndexDesc (unchanged).
• The deprecated Indexes() getter on CreateIndexRequest/CreateCollectionRequest returns by value (previously a const&). auto& v = req.Indexes(); no longer compiles and const auto& v now binds a snapshot rather than a live view; code should migrate to IndexParams(). Updating the deprecated accessors' shape is intentional so the two getters share one source of truth.
• Removing the deprecated accessors is left to a future major version.

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This pull-request has been approved by: yhmo

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[codecov]

Codecov Report

❌ Patch coverage is 97.19626% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.69%. Comparing base (a0592aa) to head (9bf2444).
⚠️ Report is 165 commits behind head on master.

Files with missing lines	Patch %	Lines
src/impl/MilvusClientV2Impl.cpp	90.00%	2 Missing ⚠️
src/impl/utils/IndexUtils.h	90.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           master     #608       +/-   ##
===========================================
+ Coverage   53.47%   89.69%   +36.22%     
===========================================
  Files          52      391      +339     
  Lines        4432    16722    +12290     
  Branches        0     1841     +1841     
===========================================
+ Hits         2370    14999    +12629     
+ Misses       2062     1723      -339     

Files with missing lines	Coverage Δ
src/impl/MilvusClientV2Impl.h	100.00% <ø> (ø)
...mpl/request/collection/CreateCollectionRequest.cpp	97.02% <100.00%> (ø)
src/impl/request/index/CreateIndexRequest.cpp	100.00% <100.00%> (ø)
src/impl/types/IndexParam.cpp	100.00% <100.00%> (ø)
...ilvus/request/collection/CreateCollectionRequest.h	100.00% <ø> (ø)
.../include/milvus/request/index/CreateIndexRequest.h	100.00% <ø> (ø)
src/impl/utils/IndexUtils.h	90.00% <90.00%> (ø)
src/impl/MilvusClientV2Impl.cpp	88.27% <90.00%> (ø)

... and 400 files with indirect coverage changes

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