estuary-cdk: improve HTTP connection timeout retry logic

[Alex-Bair]

Description:

My earlier attempt at retrying connection timeout errors in #3065 wasn't thorough enough. Connectors still encountered ConnectionTimeoutErrors when establishing the connection in request_stream and request_lines, and asyncio.TimeoutErrors when reading the response body in request. After digging into the specific errors further, I learned that:

• aiohttp.client_exceptions.ConnectionTimeoutError is raised when establishing the connection. It is not raised after the connection is established and we've begun receiving chunks from the server.
• asyncio.TimeoutError is raised when the aiohttp client timeout is reached after we've started reading chunks of the body but before we've received the final chunk.

With this knowledge, my previous assertion in e7c8c7d about it not being safe to retry timeout errors in request_stream and request_lines isn't precisely right. When we want to stream a response body, it's safe to retry timeouts when establishing the connection but not once we return the headers and body generator. Since aiohttp.client_excepitons.ConnectionTimeoutError is a subclass of asyncio.TimeoutError, and request_stream/request_lines establish the HTTP connection but don't iterate through the body, it's safe to use a general _retry_on_timeout wrapper for all of HTTPSession's public methods to retry any asyncio.TimeoutErrors. This means that all of the public methods retry connection establishment errors, but only request retries timeout errors when processing the response body.

Workflow steps:

(How does one use this feature, and how has it changed)

Documentation links affected:

(list any documentation links that you created, or existing ones that you've identified as needing updates, along with a brief description)

Notes for reviewers:

Tested behavior with a simple script. Confirmed:

• All HTTP methods retry aiohttp.client_exceptions.ConnectionTimeoutErrors and asyncio.TimeoutErrors before consuming the body.
• Only the request method retries asyncio.TimeoutErrors while reading the response body.
• Retries are attempted up to 3 times, then errors are bubbled up if there are 4 consecutive timeout errors.

I also cleaned up a couple unused imports & variables in http.py while I was in there.

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This change is 

[williamhbaker]

LGTM