fix(interactions): route interleaved function-call deltas by step index

[jstar0]

Problem

The Interactions streaming converter currently attaches arguments_delta and step.stop events to state.parts[-1] instead of the function-call step identified by event.index. When multiple function-call steps are interleaved, deltas for an earlier step are appended to the most recently started call, which can swap or merge function names and arguments in the final response.

Reproduction

Start two function-call steps at indexes 0 and 1, then send arguments_delta events for index 0 and index 1 before sending step.stop for both. The current converter routes both deltas through the last function call rather than the matching step.

Proposed fix

Track function-call parts by step index and use that mapping for arguments_delta and step.stop, while retaining the existing last-part fallback for events without an index.

Verification

A focused regression covering two interleaved calls passes with the proposed change, and the full Interactions utility test file passes 143 tests on current main.

Would maintainers like this fix submitted as a pull request?

[surajksharma07]

@jstar0 With two interleaved function-call steps arguments_delta keeps landing on state.parts[-1] instead of the step it actually belongs to so an earlier call finalizes with no args or you get a JSON_PARSE_ERROR from the concatenated string. The index-based routing in #6834 fixed it in my testing, both calls came back with correct args.

Could you try that patch on your side and confirm it holds before we push for merge?