p2.2: cap embed inputs at 3000 runes, retry too-large 500

Link-heavy chunks estimated under EmbedMaxTok still overflow
Dixie's 2048-token embed slot. Cap every embed input at 3000
runes. A 500 whose body contains 'too large' halves inputs and
retries once. Fallback logs the truncated upstream body.

internal/digest/digest_test.go

@@ -11,6 +11,7 @@ import (
 	"sync/atomic"
 	"testing"
 	"time"
+	"unicode/utf8"
 )
 
 func sseHandler(t *testing.T, reply string, saw *atomic.Pointer[chatReq], disconnected *atomic.Bool) http.Handler {
@@ -289,3 +290,47 @@ func TestEmbedUsesConfiguredModel(t *testing.T) {
 		t.Fatalf("default model %q", got)
 	}
 }
+
+func TestEmbedTooLargeRetriesAndCompletes(t *testing.T) {
+	var n500 atomic.Int32
+	var saw atomic.Pointer[chatReq]
+	var disc atomic.Bool
+	mux := http.NewServeMux()
+	mux.HandleFunc("/embed", func(w http.ResponseWriter, r *http.Request) {
+		var body struct {
+			Input []string `json:"input"`
+		}
+		_ = json.NewDecoder(r.Body).Decode(&body)
+		for _, s := range body.Input {
+			if utf8.RuneCountInString(s) > 1500 {
+				n500.Add(1)
+				w.WriteHeader(500)
+				_, _ = w.Write([]byte("input (2064 tokens) is too large to process. (current batch size: 2048)"))
+				return
+			}
+		}
+		var data []map[string]any
+		for i := range body.Input {
+			data = append(data, map[string]any{"index": i, "embedding": []float64{1, float64(i)}})
+		}
+		_ = json.NewEncoder(w).Encode(map[string]any{"data": data})
+	})
+	mux.Handle("/chat", sseHandler(t, "ok from doc", &saw, &disc))
+	ts := httptest.NewServer(mux)
+	defer ts.Close()
+	e := &Engine{
+		ChatURL: ts.URL + "/chat", EmbedURL: ts.URL + "/embed",
+		Gate: NewGate(2, 8, time.Second), Client: ts.Client(),
+	}
+	cleaned := strings.Repeat("https://example.com/path ", 4000)
+	r := e.Run(context.Background(), cleaned, "what is this?", "https://ex.test/pypi")
+	if r.Fallback != "" {
+		t.Fatalf("fallback %s text %q", r.Fallback, r.Text)
+	}
+	if n500.Load() < 1 {
+		t.Fatal("want a too-large 500 then retry")
+	}
+	if !strings.Contains(r.Text, "ok") {
+		t.Fatalf("text %q", r.Text)
+	}
+}