Implement OpenAIResponsesModel.base_url property

[chasefarmer-pixee]

Simple patch to implement the base_url property method for the OpenAIResponsesModel subclass.

[DouweM]

@chasefarmer-pixee Thanks Chase! To fix coverage you can add/modify a test like this one:

pydantic-ai/tests/models/test_openai.py

Lines 97 to 101 in e2ba27a

	def test_init():
	m = OpenAIChatModel('gpt-4o', provider=OpenAIProvider(api_key='foobar'))
	assert m.base_url == 'https://api.openai.com/v1/'
	assert m.client.api_key == 'foobar'
	assert m.model_name == 'gpt-4o'

[DouweM]

@chasefarmer-pixee Please have a look at the failing lint job; I suggest running make format before committing :)