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

[chelsealong]

Summary

Fixes #6832.

The Interactions streaming converter attached arguments_delta and step.stop
events to state.parts[-1] (the most recently started part) instead of the
function-call step identified by the event's index. When multiple
function-call steps are interleaved, argument deltas for an earlier step were
appended to the last-started call, concatenating their raw JSON strings. At
step.stop this produces invalid JSON (e.g. {"a": 1}{"b": 2} →
JSON_PARSE_ERROR), and the earlier call is finalized with no arguments —
effectively swapping/merging function names and arguments in the final
response.

Fix

• Add an fc_parts_by_index mapping to _StreamState, populated at
  StepStart from the step's index.
• Route arguments_delta and step.stop to the matching function-call part
  via a small _resolve_streaming_function_call_part helper.
• Retain the existing "last started function call" behavior as a fallback for
  events that carry no index, so single-call streaming is unchanged.

Testing Plan

• Added test_interleaved_function_call_streaming_routes_by_index, which
  starts two calls (get_weather at index 0, get_time at index 1), interleaves
  their argument deltas, then stops both, asserting each call finalizes with its
  own arguments.
• Verified the test fails without the fix: reverting only the source change
  makes the new test fail with get_weather.args == None and the log shows the
  corruption directly — arg_str: {"city": "Paris"}{"zone": "UTC"} (both deltas
  concatenated onto the last call).
• Full file passes with the fix:
  pytest tests/unittests/models/test_interactions_utils.py → 143 passed;
  including the related processor tests → 158 passed.
• Lint: pyink --check and isort --check-only both clean on the two changed
  files.

AI assistance disclosure

This change was prepared with AI assistance (Claude). A human reviewed the
diff, the reproduction, and the test evidence above before submission.