fix(openai-compatible): handle reused tool call index from Ollama

[CPIDLE]

Summary

• Fix streaming parser to handle providers (Ollama) that reuse index: 0 for all tool calls in a single response
• Add test case for parallel tool calls with reused indices

Problem

Ollama's OpenAI-compatible streaming API sends all tool calls with index: 0 instead of incrementing (0, 1, 2...). This causes the @ai-sdk/openai-compatible streaming parser to either:

1. Skip the 2nd+ tool calls (when the first has already hasFinished)
2. Merge their arguments into the first call (corrupting the JSON)

This results in a 100% failure rate on any task requiring multiple tool calls in one response.

Verified with: Ollama 0.20.2/0.20.4, models: qwen3-coder:30b, gemma4:e4b, qwen2.5-coder:7b

Ollama issue: ollama/ollama#15457

Fix

When toolCalls[index] is already occupied by a different tool call (different id), treat the incoming delta as a new tool call by assigning index = toolCalls.length.

This is backwards-compatible: providers that send correct indices are unaffected since the id will match the existing entry.

Test plan

• Added test: should handle parallel tool calls with reused index (Ollama bug)
• Verified with real Ollama streaming: dual-file write goes from 0/12 to 12/12 pass
• Existing parallel tool call tests still pass (correct indices unaffected)