Fix OpenAI AudioUrl

[alexmojaki]

Manual test:

from pydantic_ai import Agent, AudioUrl

agent = Agent(model='openai:gpt-4o-audio-preview-2024-10-01')
result = agent.run_sync([AudioUrl(url='https://cdn.openai.com/API/docs/audio/alloy.wav')])
print(result.output)
print(result.usage())

Before this PR:

pydantic_ai.exceptions.ModelHTTPError: status_code: 400, model_name: gpt-4o-audio-preview-2024-10-01, body: {'message': "Invalid value: 'audio/wav'. Supported values are: 'wav' and 'mp3'.", 'type': 'invalid_request_error', 'param': 'messages[0].content[0].input_audio.format', 'code': 'invalid_value'}

Need to add an automated test someday...

[github-actions]

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