Set VIA server/proxy result for 206 and pass-through responses (no-cache path) - #13852

Problem

HttpTransact::handle_no_cache_operation_on_forward_server_response() only records
VIA_SERVER_RESULT / VIA_PROXY_RESULT for 200 OK and 304 Not Modified. Responses
that reach the HTTPStatus::PARTIAL_CONTENT (206) or default branches are passed
through to the client without setting either slot, so at
proxy.config.http.insert_response_via_str = 2 the Via debug window is identical for a
genuine origin response and for a locally synthesized error:

HTTP/1.1 206 Partial Content
Via: http/1.1 CDN (Up [cMs f ])

HTTP/1.1 500 Cannot find server.   # DNS failure, origin never contacted
Via: http/1.1 CDN (Up [cMs f ])

Operators cannot distinguish "the origin answered 206" from "ATS synthesized a 500"
without bumping verbosity to 3.

Root cause

In the switch on the server response status, HTTPStatus::OK and
HTTPStatus::NOT_MODIFIED call SET_VIA_STRING(...), while
HTTPStatus::PARTIAL_CONTENT and default do not:

  case HTTPStatus::PARTIAL_CONTENT:
    ink_assert(s->cache_info.action == CacheAction_t::NO_ACTION);
    s->next_action = StateMachineAction_t::SERVER_READ;
    break;
  default:
    ink_assert(s->cache_info.action == CacheAction_t::NO_ACTION);
    s->next_action = StateMachineAction_t::SERVER_READ;
    break;

The cached counterpart, handle_cache_operation_on_forward_server_response(), does
set both fields in its default branch, so the no-cache path is simply inconsistent with
the cache path for the same class of response.

Fix

Set VIA_SERVER_RESULT = VIA_SERVER_SERVED and VIA_PROXY_RESULT = VIA_PROXY_SERVED
in the PARTIAL_CONTENT and default branches, matching the existing 200/304 branches in
the same function and the cached path. After the change the debug window is [cMsSf ] for
a 206 or an origin 404, and is unchanged for a DNS-failure synthesized 500 (no server
connection was made, so the server slot correctly stays empty).

This is a 4-line change using markers that already appear verbatim in the same function;
via_string[] layout, VIA_* indices and the SET_VIA_STRING macro are untouched.

Testing

• Confirmed the affected branches in src/proxy/http/HttpTransact.cc for the master
  branch: PARTIAL_CONTENT and default set neither slot before the change.
• The added lines are byte-for-byte identical to the existing calls in the OK branch
  (same function) and the cached path's default branch, so no new symbols or headers
  are introduced.
• Behaviour verified by inspection of insert_via_header_in_response(): the
  insert_response_via_string > 1 window covers VIA_CACHE .. VIA_CACHE_FILL_ACTION,
  which includes VIA_SERVER_RESULT.

Fixes #13118

Note: the linked issue was filed against 9.2.x; the same branch is also affected on
master. I opened this against master per CONTRIBUTING.md and am happy to prepare a
9.2.x backport if the fix is accepted.