fix: handle non-list response_stream in HttpResponse.json property

[bardusco]

Description

When using the aiohttp backend and Google API returns a 500 Internal Server Error, the HttpResponse.json property raises TypeError: 'ClientResponse' object is not subscriptable instead of allowing proper error handling.

This happens because response_stream can be an aiohttp.ClientResponse object (not a list) when an error occurs, and the code attempts to access response_stream[0] without checking the type first.

Root Cause

In google/genai/_api_client.py, the json property assumes response_stream is always a list:

@property
def json(self) -> Any:
    if not self.response_stream[0]:  # Empty response  <-- fails here
        return ''
    return self._load_json_from_response(self.response_stream[0])

The Fix

Add a type check before attempting list access:

@property
def json(self) -> Any:
    if not isinstance(self.response_stream, list):
        return None
    if not self.response_stream or not self.response_stream[0]:
        return ''
    return self._load_json_from_response(self.response_stream[0])

Impact

Without this fix:

• The original ServerError: 500 INTERNAL is masked by TypeError
• Retry logic cannot identify the error as transient
• Error handling and recovery is broken

With this fix:

• The ServerError is raised correctly
• Retry logic can identify transient errors and retry
• Proper error messages are logged

Testing

This fix has been validated in production for 24+ hours:

Before the fix (Dec 30, 2025 until 18:40 UTC-3):

• Multiple TypeError: 'ClientResponse' object is not subscriptable errors
• Messages lost due to masked errors
• Retry logic failing

After the fix (Dec 30, 2025 19:00 UTC-3 - Dec 31, 2025 16:00 UTC-3):

• Zero TypeError: 'ClientResponse' object is not subscriptable errors
• 50+ transient errors correctly identified and retried
• Zero message handler failures
• All retries successful

Related Issue

Fixes #1897