Bug: Add missing name field to ResponseFunctionCallArgumentsDoneEvent

[Krivoblotsky]

What

The response.function_call_arguments.done streaming event is documented by OpenAI as carrying a name field identifying the function whose arguments have been finalized:

{
  "type": "response.function_call_arguments.done",
  "item_id": "item-abc",
  "name": "get_weather",
  "output_index": 1,
  "arguments": "{ \"arg\": 123 }",
  "sequence_number": 1
}

The generated Components.Schemas.ResponseFunctionCallArgumentsDoneEvent is missing it (the upstream OpenAPI spec lags), so SDK users couldn't read the function name from this event.

This PR:

• Extracts the type to Sources/OpenAI/Public/Schemas/Edited/ResponseFunctionCallArgumentsDoneEvent.swift with a new public var name: Swift.String? field, following the documented pattern in CONTRIBUTING.md ("Extracting a generated Type") and the existing precedent for ResponseMCPCallArgumentsDoneEvent.
• Updates the ResponseStreamEvent facade so .functionCallArguments(.done(_)) carries the extracted type. Adds an early decode block (mirroring the MCP one) so JSON with name round-trips correctly. Preserves the existing rawEvent.value19 fallback by converting from the generated type to the new one (with name: nil).

Why

Closes #409. Function-call routing on the Responses API requires knowing which function the arguments belong to. Without name, consumers had to track it externally via the surrounding output_item.added event, which is brittle.

Affected Areas

• New file: Sources/OpenAI/Public/Schemas/Edited/ResponseFunctionCallArgumentsDoneEvent.swift
• Modified: Sources/OpenAI/Public/Schemas/Facade/ResponseStreamEvent.swift
  • Enum case type changed from Schemas.ResponseFunctionCallArgumentsDoneEvent to the new top-level ResponseFunctionCallArgumentsDoneEvent.
  • Added an early-decode block before the rawEvent fallback.
  • The rawEvent.value19 branch now bridges to the extracted type so the fallback path stays valid.
• New tests: Tests/OpenAITests/ResponseFunctionCallArgumentsDoneEventTests.swift — decode-with-name, decode-without-name, round-trip, and an end-to-end check through ResponseStreamEvent.

More Info

• Source-compat note: pattern-matched code using .functionCallArguments(.done(let event)) continues to compile (all the original fields are still present on the new type); only the static type of event changes. Code that explicitly annotated the bound variable as Components.Schemas.ResponseFunctionCallArgumentsDoneEvent would need to drop the annotation or switch to the new type.
• name is modeled as optional String? for forward-compat with servers that haven't shipped the field yet.
• Local: swift build clean; full swift test is 183 passed / 0 failed (4 new tests for this event).

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