fix(http): reject HTTP/1 responses with non-chunked final Transfer-Encoding - #1109

Addresses #1042

Summary

• Reject HTTP/1 downstream responses when Transfer-Encoding is present but chunked is not the final transfer coding, in accordance with RFC 9112 §6.3.
• Update test_response_multiple_te_headers_chunked_not_last and add test_response_single_te_header_not_chunked regression tests.

Motivation

Per RFC 9112 §6.3:

"If a Transfer-Encoding header field is present in a response and the chunked transfer coding is not the final encoding, the message body length cannot be determined reliably; the recipient MUST parse or discard the response as an error."

While the server side (HttpSession::validate_request) already enforces !self.is_chunked_encoding(), HttpSession::validate_response in protocols/http/v1/client.rs lacked this check, leading to inconsistent framing handling and potential request/response desynchronization risks.

Design

• In HttpSession::validate_response: if resp_header.headers.contains_key(header::TRANSFER_ENCODING) and !self.is_chunked_encoding(), immediately return Error::e_explain(InvalidHTTPHeader, "non-chunked final Transfer-Encoding").
• Align client-side behavior with existing server-side validation.

Tests

• Updated test_response_multiple_te_headers_chunked_not_last to assert InvalidHTTPHeader error.
• Added test_response_single_te_header_not_chunked to verify single non-chunked TE headers.
• All 83 pingora-core HTTP/1 client unit tests pass.