Missing aiohttp version constraint causes AttributeError for ClientConnectorDNSError

## Description

The `google-genai` package uses `aiohttp.ClientConnectorDNSError` in `_api_client.py`, but does not declare a minimum version constraint for `aiohttp`. This causes an `AttributeError` when an older version of `aiohttp` is installed transitively through another dependency.

`ClientConnectorDNSError` was introduced in aiohttp 3.10.11 (see [aiohttp changelog](https://docs.aiohttp.org/en/stable/changes.html#id24)).

## Error

```
  File "/home/ray/anaconda3/lib/python3.11/site-packages/google/genai/_api_client.py", line 1242, in _async_request_once
    aiohttp.ClientConnectorDNSError,

  File "/home/ray/anaconda3/lib/python3.11/site-packages/aiohttp/__init__.py", line 240, in __getattr__
    raise AttributeError(f"module {__name__} has no attribute {name}")

AttributeError: module aiohttp has no attribute ClientConnectorDNSError
```

## Expected Behavior

The package should explicitly declare `aiohttp>=3.10.11` (or `>3.10.10`) in its dependencies to ensure the required `ClientConnectorDNSError` class is available.

## Reproduction

1. Install `google-genai` in an environment where `aiohttp<3.10.11` is already installed as a transitive dependency
2. Make an async API call that triggers the retry logic in `_api_client.py`
3. The import of `aiohttp.ClientConnectorDNSError` fails

## Suggested Fix

Add a minimum version constraint to the `aiohttp` dependency in `pyproject.toml`:

```toml
aiohttp = ">=3.10.11"
```

## Environment

- Python 3.11
- google-genai 1.60.0
- aiohttp < 3.10.11 (installed transitively)

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