fix: use async clients for acompletion methods

[whoisnnamdi]

Problem

Running async code paths (e.g., REPL chunk investigation) produced:
Error: object ChatCompletion can't be used in 'await' expression

Changes

• Fixed acompletion() methods in OpenAI and Portkey clients that were incorrectly awaiting synchronous client responses
• Added AsyncOpenAI and AsyncPortkey clients for proper async support, matching existing Anthropic client pattern

Specifically:

• rlm/clients/openai.py: Initialize self.async_client with openai.AsyncOpenAI(), use in acompletion()
• rlm/clients/portkey.py: Initialize self.async_client with AsyncPortkey(), use in acompletion()

The Anthropic client already followed this pattern correctly with both anthropic.Anthropic() and anthropic.AsyncAnthropic(). OpenAI and Portkey now match.

[alexzhang13]

Thanks, this is a good catch. In general I should add better testing for the asynchronous calls, it wasn't tested very heavily.