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