[rn7s2]

A plain-HTTP provider could not be reached on any machine that exports
http_proxy, and the failure said nothing about why: every request came back
HTTP 400: with no body and no tokens spent (input 0).

Why

evo deliberately resolves a proxy per request (env-proxy, honouring the
lowercase http_proxy that dexador's own *default-proxy* misses) and hands
it to dexador as :proxy. dexador then connects to the proxy but writes an
origin-form request line:

POST /v1/messages HTTP/1.1
Host: 192.168.1.5:3425

where an HTTP proxy requires the absolute form
(POST http://192.168.1.5:3425/v1/messages HTTP/1.1). xray answers that with
HTTP/1.1 400 Bad Request and Content-Length: 0 — the empty body the agent
reported. An HTTPS provider is unaffected, since HTTPS tunnels with CONNECT.

What

:proxy on register-provider now decides in place of the environment:

• a proxy URL string forces that one;
• nil reaches this provider directly, whatever http_proxy/https_proxy
  say — the case of a plain-HTTP endpoint behind a machine-wide proxy;
• absent keeps the environment's answer, exactly as before.

with-proxy takes the override as an optional third element and makes it the
proxy dexador uses, so it is not undone by a proxy baked into dexador at build
time. perform-request carries it (a defgeneric lambda list may not give a
&key default, so the method does), and call-provider passes the provider's
registered value. provider-config publishes :proxy only when it was
registered: an absent key and a nil one mean different things.

Verification

• make test unit suite: 2087 passed, 0 failed, including new checks that
  an override beats the environment, and that absent is not nil in the
  resolved config.
• Same endpoint, same process, two provider keys — one registered :proxy nil,
  one not, with http_proxy set:

:magpie-direct -> stop-reason=:STOP  error=NIL       content=("Hi!")
:magpie-env    -> stop-reason=:ERROR error="HTTP 400: "

• The built binary, init.lisp registering the same provider, prompt
  Reply with exactly: proxy override works:

with :proxy nil   -> proxy override works
without           -> ✗ provider error: HTTP 400: