fix: OpenAI Responses API parallel tool calls losing call_ids (#9659)

When GPT-5 makes parallel tool calls, each function_call has a unique
call_id (fc_*). These IDs were being overwritten instead of accumulated,
causing the error: "No tool call found for function call output with call_id"

Changes:
- sessionSlice.ts: Accumulate IDs in responsesOutputItemIds[] array
- openaiTypeConverters.ts: Emit function_call for EACH toolCall with matching ID

Ref: https://platform.openai.com/docs/guides/function-calling

Author: houssemzaier