fix(openai): remove unreachable None type assignments for OpenAI classes (#1546)

The second try/except block that assigns None to AsyncOpenAI,
AsyncAzureOpenAI, AzureOpenAI, and OpenAI is unreachable because:
1. The first try/except raises ModuleNotFoundError if openai is missing
2. Line 28 already imports from openai._types (v1.x only), so if
   execution reaches the second block, openai 1.x is installed

Merge both imports into a single try/except so type checkers correctly
infer these as their actual types instead of type | None.

Co-authored-by: Hassieb Pakzad <68423100+hassiebp@users.noreply.github.com>