Fix working behind http proxy

[maxnilz]

Initial Checks

• I confirm that I'm using the latest version of Pydantic AI
• I confirm that I searched for my issue in https://github.com/pydantic/pydantic-ai/issues before opening this issue

Description

When creating the HTTP client in infer_provider, it specifies a transport that has no proxy used explicitly, which causes the HTTP request to the LLM provider to be unreachable behind a proxy, e.g., the completion request.

Python, Pydantic AI & LLM client version

Python: 3.12
Pydantic-ai: the latest main branch
LLM cient version: any