proxy: chunked response, error code

[icing]

Add test1715 to check proper handling of chunked transfer encoding in CONNECT responses. Change proxy error code from 56 (RECV_ERROR) for everything to 7 (COULDNT_CONNECT) when the server response could be read successfully, but establishing the connection is not possible (http status code wrong).

Adapt several test expectations from 56 to 7.

[bagder]

augment review

[augmentcode]

🤖 Augment PR Summary

Summary: Improves proxy CONNECT tunnel error reporting and adds coverage for tricky CONNECT responses.

Changes:

• Adjust HTTP/1 CONNECT response draining to prioritize Transfer-Encoding: chunked over Content-Length when ignoring a response body.
• Change CONNECT-tunnel failures with a fully readable non-2xx response to return CURLE_COULDNT_CONNECT (HTTP/1 and HTTP/2 proxy filters).
• Add a new regression test (test1715) and update existing test expectations (including Python tests) to match the new exit code semantics.

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[augmentcode]

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