fix: correlate id-less parallel tool-call fragments by stream index

[ege-arhan]

Problem

OpenAI streamed tool-call deltas carry a stable index, while id is optional on continuation fragments. Continue's fromChatCompletionChunk() dropped tool_call.index, and applyToolCallDelta() fell back to updating the most recently tracked tool call whenever a fragment had no id. With two parallel tool calls whose argument fragments interleave, one call's arguments were applied to the other call.

Fixes #13223.

Solution

• Preserve tool_call.index through ToolCallDelta (new optional index?: number on the type + converter passthrough).
• In applyToolCallDelta(), when a delta has no id but has an index, match the existing tool-call state by stream position instead of "last array element".
• Existing id-based correlation stays first choice; the positional fallback remains for providers supplying neither field.

Impact

• Parallel tool-call streaming attributes argument fragments to the correct call.
• Single-call streaming behavior unchanged (id path and last-element fallback untouched).
• Affects OpenAI/OpenAI-compatible provider stream path only.

Evidence

• New regression test in core/llm/openaiTypeConverters.test.ts asserting index survives fromChatCompletionChunk().
• Standalone repro of the issue's 4-delta sequence (index=0/id=call_A, index=1/id=call_B, index=0 id-less, index=1 id-less): after fix, call_A args = '{"target":"A_ONLY"}', call_B args = '{"target":"B_ONLY"}' (PASS).
• Full npm install in this sandbox failed on /tmp disk quota (chromium-bidi tar write), so the repo vitest suite could not run here; test added for CI.

Fixes #13223

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[ege-arhan]

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