fix(openai): responses function_call mapping

[Quazistax]

Maybe fixes #8773 , with 2nd commit might help with #8933 too (I tried it with VS Code, not with JetBrains)

Description

• to handle function_call messages loaded from session file
• use call_id as item id if response id is not available
• ensure function_call item id is prefixed with fc

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Summary by cubic

Fixes function_call mapping in OpenAI responses to emit valid items from session-loaded messages with stable IDs. When multiple tool calls follow a reasoning message, synthesizes a single orchestrator function_call to avoid protocol violations. Addresses #8773.

• Bug Fixes
  • Emit function_call items from toolCalls using respId, reasoningId, or call_id; prefix IDs with fc_ and index when multiple.
  • Synthesize a "multi_tool_use" orchestrator call after reasoning, wrapping all tool uses and preserving original call_ids/args.
  • Fallback to an assistant message when no toolCalls exist but respId is present.

^{Written for commit 7ba2d46. Summary will update automatically on new commits.}

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Reviewed the changes - no documentation updates needed. This PR fixes internal OpenAI Responses API type conversion logic for function_call messages loaded from session files. The changes are purely implementation details that don't affect any user-facing features, configuration options, or documented behavior.

[Quazistax]

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[cubic-dev-ai]

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