fix(proxy): address review findings on admission control (#64)

- tracing: responseWriter now forwards Flush and exposes Unwrap. Without it
  w.(http.Flusher) failed for every request passing the tracing middleware, so
  the parked-stream keep-alive never started: a parked streaming request wrote
  nothing for up to ADMISSION_MAX_PARK_SECONDS, which is exactly the client
  timeout the keep-alive was added to prevent.
- proxy: once the SSE status line is spent, ModifyResponse frames an upstream
  response >= 400 as an event: error instead of letting a raw error body land
  inside the stream, where a streaming client reads it as an empty completion
  and reports success. net/http keeps the first status, so the upstream status
  cannot be forwarded after the park has written one.
- engine_state: new Snapshot() returns the state and its tick channel under one
  lock. Get and Tick taken separately can miss the poll that lands between the
  two calls, parking a request for an extra interval.
- engine_state: observer_degraded now tracks current parsability rather than
  being raised once on the first poll and never cleared, which pinned it at 1
  after a single truncated response and never reported drift that appeared
  later. markDegraded only touches the exporter when the value changes.
- deployment: ADMISSION_MAX_INFLIGHT is set explicitly to 4, documenting the
  coupling to the engine's --max-num-seqs, instead of silently defaulting to
  VLLM_MAX_NUM_SEQS.

internal/tracing/middleware.go

@@ -36,6 +36,21 @@ func (rw *responseWriter) Write(b []byte) (int, error) {
 	return n, err
 }
 
+// Flush forwards to the wrapped writer so that streaming handlers can still
+// detect http.Flusher through this wrapper. Without it a bare
+// `w.(http.Flusher)` assertion fails for every request that passes through
+// this middleware, which silently disables SSE flushing and keep-alives.
+func (rw *responseWriter) Flush() {
+	if f, ok := rw.ResponseWriter.(http.Flusher); ok {
+		f.Flush()
+	}
+}
+
+// Unwrap exposes the wrapped writer to http.ResponseController.
+func (rw *responseWriter) Unwrap() http.ResponseWriter {
+	return rw.ResponseWriter
+}
+
 // Middleware returns an HTTP middleware that adds tracing to requests.
 func Middleware(next http.Handler) http.Handler {
 	tracer := otel.Tracer(tracerName)