Fixes #3005.

This PR tries implementing a possible solution to #3005 by adding a new run_stream_sync method that uses run_stream under the hood, based on the run_sync implementation that uses run internally.

Currently, it uses an 'eager' approach by loading the async iterable upfront and then providing normal iterable to access the stream. This has an impact on latency as it is not true streaming, but it avoids some opaque race conditions that can appear in a 'lazy' implementation.

I'd like to get some feedback on whether this approach is worthwhile or if it would be better to try and get a working 'lazy' implementation. My thinking is that, if latency is a concern, it would probably be best to point users towards the async version regardless, leaving the sync methods as conveniences for certain cases.

from pydantic_ai import Agent

agent = Agent('openrouter:openai/gpt-4o')

with agent.run_stream_sync('What are the capitals of France and Germany?') as response:
    for text in response.stream_text():
        print(text)

# The capital of France is Paris, and the capital
# The capital of France is Paris, and the capital of Germany is Berlin
# The capital of France is Paris, and the capital of Germany is Berlin.