fix: remove unsupported max_line_length arg from aiohttp readline()

[SeleneXX]

Fixes #2487

Regression introduced in #2437.

Problem:
google-genai 2.5.0 crashes when streaming with aiohttp installed:
TypeError: StreamReader.readline() got an unexpected keyword argument 'max_line_length'

Root cause:
aiohttp.StreamReader.readline() does not accept any parameters. Its signature is:

async def readline(self) -> bytes:
    return await self.readuntil()

The max_line_length keyword argument passed in _api_client.py is invalid for all released versions of aiohttp.

Why this is safe: The line-length limit is already governed by the limit parameter of StreamReader, which is set correctly via read_bufsize=READ_BUFFER_SIZE when the aiohttp ClientSession is created.

Change: Remove the unsupported max_line_length keyword argument from the readline() call.

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