Pass through fetch error messages in APIConnectionError

[cnaples79]

Summary

• include the underlying fetch error message when constructing APIConnectionError
• assert the network error test surfaces the original message in addition to the error type

Rationale

• fixes openai/openai-node#1656 so callers receive the full connection failure details instead of the generic "Connection error"

Changes

• update the retry logic to pass the error message into APIConnectionError
• extend the streaming network error test to check the message is preserved

Fixes #1656