Add connect_down_policy: 4 for empty reply

[masaori335]

Problem

When an origin server accepts a TCP/TLS connection but closes it without sending any HTTP response, ATS does not mark the origin as down — even after exhausting all retry attempts. This can cause ATS to keep hammering a misbehaving origin on every request.

Fix

• Add a new policy, 4, all failures covered by policy 3, plus the case where server_response_hdr_bytes == 0 at the end of a transaction — i.e. the origin closed or reset the connection after the TCP handshake without sending any HTTP response.
• The HttpSM::track_connect_fail() function was also refactored for clarity.
• AuTest uses new on_connect: refuse|reset feature (thank you, @bneradt )

notes

Two cases are explicitly excluded:

• Reused keep-alive connections: there is a known race between ATS reusing a keep-alive connection and the origin closing it, described by @moonchen on Error messages when server closes connection #12437. Considering this case, the policy 4 is only applied for the first transaction.
• Multiplexed origins ( HTTP/2 ): per-stream error observation (e.g. RST_STREAM) and does not indicate the underlying connection is unhealthy.