fix: Add aiohttp conditional type checking

[ronantakizawa]

Add aiohttp conditional type checking.

• Problem: _aiter_response_stream() method referenced aiohttp.ClientResponse without checking if aiohttp
  was available, causing NameError when aiohttp wasn't installed
• Solution: Added conditional type checking that only includes aiohttp types when has_aiohttp is True

Before the Fix:
This would ALWAYS try to reference aiohttp.ClientResponse
if not isinstance(self.response_stream, (httpx.Response, aiohttp.ClientResponse)):
❌ CRASH: NameError if aiohttp not installed

After the Fix:
Step 1: Build valid types conditionally
valid_types = (httpx.Response,) Always start with httpx
if has_aiohttp:
valid_types = (httpx.Response, aiohttp.ClientResponse) Add aiohttp if available
Step 2: Use the conditional types
if not isinstance(self.response_stream, valid_types):
✅ No crash - only references types that exist

Fixes issue #1230