fix(openai): strict OpenAI wire compatibility — Open WebUI can chat again

/v1/chat/completions answered ordinary client payloads with a bare HTTP 422
(a plain-text body no OpenAI client can parse), so Open WebUI showed neither
an answer nor an error — it just hung. Parse the body leniently instead:
content as string/null/content-part array, tools: null, stream: null, a
string reasoning_effort, max_completion_tokens, messages[].name and
tool_call_id, and a missing Content-Type. Unreadable bodies now come back as
an OpenAI error envelope with 400.

Responses are strict OpenAI JSON: no "tool_calls": null, finish_reason mapped
to the spec's closed set (cortiq serve's max_tokens -> length), never an empty
id, and `model` echoes the id the client requested — the model that actually
answered stays in cortiq.selected_model and X-Cortiq-Selected-Model.

Streaming is re-normalised rather than forwarded verbatim: a leading role
delta, message-shaped chunks converted to delta, one terminal finish_reason,
always a trailing data: [DONE], and an SSE error event when the upstream dies
mid-flight. Adds X-Accel-Buffering: no so a fronting proxy cannot buffer the
stream into a hang.

GET /v1/models now lists managed [[cmf.servers]] models with their live
running/healthy state, so clients can pin a local .cmf directly.

Also strips client bookkeeping keys (chat_id, session_id, metadata, features,
...) from the upstream call and drops the leftover [to_wire] debug prints.

Cargo.lock

@@ -285,7 +285,7 @@ checksum = "1d07550c9036bf2ae0c684c4297d503f838287c83c53686d05370d0e139ae570"
 
 [[package]]
 name = "cortiq-gateway"
-version = "0.2.44"
+version = "0.2.45"
 dependencies = [
  "anyhow",
  "arc-swap",