fix: strip hop-by-hop headers from bidirectional response

[evgenizb]

Forwarding cm2's Transfer-Encoding/Connection/Keep-Alive headers back through the Lambda Function URL caused Step Functions HTTP Invoke to hang for 60s waiting for chunked-stream framing it does not support. Filter the RFC 7230 hop-by-hop set plus Content-Length/Content-Encoding before send_task_success.

[gemini-code-assist]

Code Review

This pull request introduces a mechanism to filter out hop-by-hop headers from proxy responses by defining a HOP_BY_HOP_HEADERS set and a filter_response_headers helper function. The feedback suggests adding the common 'proxy-connection' header to the filtered set to prevent potential connection issues, and adding a defensive check to handle cases where the input headers might be None or empty.

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[gemini-code-assist]

The non-standard but widely used Proxy-Connection header behaves similarly to the standard Connection header and is often sent by older proxies or clients. To prevent potential hanging or connection issues, it is safer to include "proxy-connection" in the set of hop-by-hop headers to be filtered out.

Suggested change

	HOP_BY_HOP_HEADERS = frozenset(
	{
	"connection",
	"keep-alive",
	"proxy-authenticate",
	"proxy-authorization",
	"te",
	"trailer",
	"transfer-encoding",
	"upgrade",
	"content-length",
	"content-encoding",
	}
	)
	HOP_BY_HOP_HEADERS = frozenset(
	{
	"connection",
	"keep-alive",
	"proxy-authenticate",
	"proxy-authorization",
	"proxy-connection",
	"te",
	"trailer",
	"transfer-encoding",
	"upgrade",
	"content-length",
	"content-encoding",
	}
	)

[gemini-code-assist]

To ensure robustness and adhere to defensive programming practices, it is safer to handle cases where headers might be None or empty (for example, in unit tests or mock environments) by returning an empty dictionary early.

Suggested change

	def filter_response_headers(headers):
	return {k: v for k, v in headers.items() if k.lower() not in HOP_BY_HOP_HEADERS}
	def filter_response_headers(headers):
	if not headers:
	return {}
	return {k: v for k, v in headers.items() if k.lower() not in HOP_BY_HOP_HEADERS}