schedule,shared: move concurrency 429 limits into scheduler code (#849)

- make concurrency limiting the scheduler.Scheduler's responsibility
- eliminate the separate concurrency limit middleware 
- move concurrencyLimit logic into scheduler.FIFO to maintain backwards compatibility
- add HTTPError from #834 

Updates #834

internal/shared/http.go

@@ -37,6 +37,16 @@ var (
 )
 
 func SendError(w http.ResponseWriter, r *http.Request, err error) {
+	var httpErr HTTPError
+	if errors.As(err, &httpErr) {
+		for k, v := range httpErr.Header() {
+			w.Header()[k] = v
+		}
+		w.WriteHeader(httpErr.StatusCode())
+		w.Write(httpErr.Body())
+		return
+	}
+
 	switch {
 	case errors.Is(err, ErrNoModelInContext):
 		SendResponse(w, r, http.StatusNotFound, "no model id could be identified")

internal/shared/httperror.go

@@ -0,0 +1,63 @@
+package shared
+
+import (
+	"encoding/json"
+	"net/http"
+	"strconv"
+)
+
+// HTTPError is an error that carries a complete HTTP response. A producer (e.g.
+// a scheduler shedding a request) returns one of these; a renderer (e.g.
+// router.SendError) writes the status, headers, and body verbatim instead of
+// mapping the error to a generic status. It is the seam that lets a component
+// shed a request with a rich response (e.g. a 429 with rate-limit headers and a
+// JSON hint body) without the renderer knowing the producer's internals.
+type HTTPError interface {
+	error
+	StatusCode() int
+	Header() http.Header
+	Body() []byte
+}
+
+// ConcurrencyLimitError is an HTTPError for a 429 concurrency-limit rejection.
+// Zero-value fields fall back to sensible defaults: a 1-second Retry-After and a
+// JSON hint body.
+type ConcurrencyLimitError struct {
+	// RetryAfter, when > 0, is sent as the Retry-After header (in seconds).
+	// Defaults to 1.
+	RetryAfter int
+
+	// Message overrides the JSON body's "error" field. Defaults to
+	// "Too many requests".
+	Message string
+}
+
+func (e ConcurrencyLimitError) Error() string { return "concurrency limit reached" }
+
+func (e ConcurrencyLimitError) StatusCode() int { return http.StatusTooManyRequests }
+
+func (e ConcurrencyLimitError) Header() http.Header {
+	h := http.Header{}
+	h.Set("Content-Type", "application/json")
+	h.Set("Retry-After", e.retryAfter())
+	return h
+}
+
+func (e ConcurrencyLimitError) Body() []byte {
+	b, _ := json.Marshal(map[string]string{"error": e.message()})
+	return b
+}
+
+func (e ConcurrencyLimitError) retryAfter() string {
+	if e.RetryAfter > 0 {
+		return strconv.Itoa(e.RetryAfter)
+	}
+	return "1"
+}
+
+func (e ConcurrencyLimitError) message() string {
+	if e.Message != "" {
+		return e.Message
+	}
+	return "Too many requests"
+}