feat: add lookup_index_by_key to Rust Vector for index-based search

[statxc]

Closes: #8667

Problem

lookup_by_key performs binary search on sorted vectors but can only return the matched object. There is no way to get the element's index, which is needed when using vector positions as references (e.g., graph adjacency lists, ECS entity lookups, index-based cross-referencing between vectors).

Fix

Add lookup_index_by_key to Vector - identical binary search logic as lookup_by_key, but returns Option<usize> (the index) instead of Option<T::Inner> (the object).

Changes

• rust/flatbuffers/src/vector.rs - add lookup_index_by_key method on Vector<'a, T>
• tests/rust_usage_test/tests/integration_test.rs - add 5 test cases for the new method

Test plan

# Run new tests
cd tests/rust_usage_test && cargo test lookup_index_by_key

# Run full test suite
cd tests/rust_usage_test && cargo test

[statxc]

@jtdavis777 I'd appreciate you also review this PR.

[statxc]

Thanks for your feedback @jtdavis777 . I removed duplicated codes. Please review again

[statxc]

@jtdavis777 Any update for me, please

[statxc]

@jtdavis777 Please let me know any feedbacks. what else should I update?

[jtdavis777]

@statxc looks like there is a merge conflict in the tests. can you merge or rebase on master for me?

[statxc]

@jtdavis777 Fixed conflicts. Could you run CI testing?

[statxc]

@jtdavis777 I saw testing error. I found problem - I missed closing bracket } in integration_test.rs file while fixing conflicts.
how can I update this?

[statxc]

Should I send another pr for this? please let me know your best option

[jtdavis777]

interesting this wasn't caught by the tests themselves.. did they not run properly? can you inspect the testing logs?

Just submit a new PR, no need for a new issue just reference your comment.

[statxc]

@jtdavis777
https://github.com/google/flatbuffers/actions/runs/22933105103

please check this. it is testing error log.

I will submit new pr for this.

[jtdavis777]

ahh sure enough. Do we not have those tests marked as required?? Lovely. Thank you for being thorough, looking forward to the fix PR. I only have a little bit of time each day to review but we can get this in as soon as possible.

[statxc]

@jtdavis777 Fixed error. Please review this PR: #8969