phraser: report the failure next to the fallback (V-397)

PhraseChat and PhraseQuery returned canned text with a nil error, so a dead
or OOM-killed server was indistinguishable from bad phrasing — "не знаю." is
also a legitimate answer.

Both now return the fallback text AND the error. The daemon keeps using the
text, so the turn still survives; a measuring caller counts a real failure.
An empty response is its own error: the model is up and said nothing.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

internal/phraser/failure_test.go

@@ -0,0 +1,72 @@
+package phraser
+
+import (
+	"context"
+	"net/http"
+	"net/http/httptest"
+	"strings"
+	"testing"
+)
+
+// A dead server must be distinguishable from bad phrasing. Both PhraseChat and
+// PhraseQuery keep the turn alive with canned text — "поговорили.", "не знаю.",
+// "вот что я нашла: …" — and every one of those is also a legitimate reply, so
+// the text alone cannot say which happened. The error is the only signal, and
+// before Vikunja #397 it was dropped: the talk scorer reported a full run with
+// zero errors off a server that answered nothing.
+func TestPhrasingReportsTheFailureWithTheFallback(t *testing.T) {
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		http.Error(w, "model not loaded", http.StatusServiceUnavailable)
+	}))
+	t.Cleanup(srv.Close)
+	p := NewLLMPhraserAt(srv.URL, Config{})
+
+	cases := []struct {
+		name string
+		call func() (string, error)
+		want string
+	}{
+		{"chat", func() (string, error) {
+			return p.PhraseChat(context.Background(), "как дела", nil)
+		}, "поговорили."},
+		{"knowledge", func() (string, error) {
+			return p.PhraseQuery(context.Background(), "кто написал войну и мир", nil)
+		}, "не знаю."},
+		{"evidence", func() (string, error) {
+			return p.PhraseQuery(context.Background(), "сколько воды я выпил", []string{"два литра"})
+		}, "вот что я нашла: два литра"},
+	}
+	for _, c := range cases {
+		t.Run(c.name, func(t *testing.T) {
+			got, err := c.call()
+			if err == nil {
+				t.Fatalf("no error from a dead server; the scorer would count this as bad phrasing")
+			}
+			if got != c.want {
+				t.Errorf("fallback text = %q, want %q — the daemon still has to say something", got, c.want)
+			}
+		})
+	}
+}
+
+// An empty answer is a failure too: the server is up and produced no tokens,
+// which is not an answer and must not score as one.
+func TestEmptyKnowledgeAnswerIsAnError(t *testing.T) {
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.Header().Set("Content-Type", "application/json")
+		w.Write([]byte(`{"choices":[{"message":{"content":""}}]}`))
+	}))
+	t.Cleanup(srv.Close)
+	p := NewLLMPhraserAt(srv.URL, Config{})
+
+	got, err := p.PhraseQuery(context.Background(), "кто написал войну и мир", nil)
+	if err == nil {
+		t.Fatal("an empty response scored as an answer")
+	}
+	if got != "не знаю." {
+		t.Errorf("fallback text = %q, want \"не знаю.\"", got)
+	}
+	if !strings.Contains(err.Error(), "empty") {
+		t.Errorf("error = %v; want it to name the empty response", err)
+	}
+}