feat: expose transaction drop index through FFI

[yuruiz]

Summary

• expose Transaction::DropIndex through the C FFI
• update symbol exports and Python cffi declarations
• add an FFI regression test for dropping an index

Validation

• make build succeeded in the Milvus builder container
• git diff --check passed

[sre-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: yuruiz
To complete the pull request process, please assign tedxu after the PR has been reviewed.
You can assign the PR to them by writing /assign @tedxu in a comment when ready.

The full list of commands accepted by this bot can be found here.

Details

Needs approval from an approver in each of these files:

• OWNERS

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment

[codecov]

Codecov Report

❌ Patch coverage is 70.00000% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 76.43%. Comparing base (2120d43) to head (021db18).

Files with missing lines	Patch %	Lines
cpp/src/ffi/manifest_c.cpp	70.00%	3 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #622   +/-   ##
=======================================
  Coverage   76.43%   76.43%           
=======================================
  Files         173      173           
  Lines       17561    17571   +10     
  Branches     2653     2654    +1     
=======================================
+ Hits        13422    13431    +9     
- Misses       4139     4140    +1     

Flag	Coverage Δ
cpp	79.02% <70.00%> (+<0.01%)	⬆️
python	44.45% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.