Fix streaming function call decoding error when 'name' field is missing

[jamesrochabrun]

Summary

• Make name optional in FunctionCallArgumentsDoneEvent to fix decoding error when OpenAI API omits this field

Fixes #188

Problem

When streaming responses with function calls, the decoder fails with:

Key 'name' not found: No value associated with key CodingKeys(stringValue: "name", intValue: nil)

The OpenAI API doesn't always include the name field in response.function_call_arguments.done streaming events.

Solution

Changed name from String to String? in FunctionCallArgumentsDoneEvent, aligning with the pattern already used in ToolCall.FunctionCall.

Test plan

• Run ResponseAPIDemo with a custom function tool
• Verify streaming function calls complete without decoding errors

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