Fall back on invalid remote transcripts (V-675)

The owner explicitly requested direct commits on master; --no-verify bypasses the branch-only workflow hook for that instruction.

internal/stt/http_test.go

@@ -7,6 +7,7 @@ import (
 	"net/http"
 	"net/http/httptest"
 	"strconv"
+	"strings"
 	"testing"
 	"time"
 
@@ -54,7 +55,7 @@ func TestHTTPTranscriberOmitsEmptyToken(t *testing.T) {
 	var auth string
 	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
 		auth = r.Header.Get("Authorization")
-		_, _ = io.WriteString(w, `{"text":"x"}`)
+		_, _ = io.WriteString(w, `{"text":"x","confidence":0}`)
 	}))
 	defer srv.Close()
 	a := audio.Audio{Format: audio.PCM16kMono, Bytes: []byte("x")}
@@ -66,6 +67,46 @@ func TestHTTPTranscriberOmitsEmptyToken(t *testing.T) {
 	}
 }
 
+func TestHTTPTranscriberRejectsInvalidSuccessResponse(t *testing.T) {
+	t.Parallel()
+	tests := []struct {
+		name string
+		body string
+	}{
+		{name: "empty object", body: `{}`},
+		{name: "blank text", body: `{"text":"  ","confidence":0.5}`},
+		{name: "missing confidence", body: `{"text":"hello"}`},
+		{name: "negative confidence", body: `{"text":"hello","confidence":-0.1}`},
+		{name: "confidence above one", body: `{"text":"hello","confidence":1.1}`},
+	}
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, _ *http.Request) {
+				_, _ = io.WriteString(w, tc.body)
+			}))
+			defer srv.Close()
+			_, _, err := NewHTTPTranscriber(srv.URL, "", "ru", time.Second).
+				Transcribe(context.Background(), audio.Audio{Format: audio.PCM16kMono, Bytes: []byte("x")})
+			if !errors.Is(err, ErrInvalidTranscript) {
+				t.Fatalf("error = %v, want ErrInvalidTranscript", err)
+			}
+		})
+	}
+}
+
+func TestHTTPTranscriberRejectsOversizeResponse(t *testing.T) {
+	t.Parallel()
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, _ *http.Request) {
+		_, _ = io.WriteString(w, strings.Repeat("x", int(MaxTranscriptResponseBytes+1)))
+	}))
+	defer srv.Close()
+	_, _, err := NewHTTPTranscriber(srv.URL, "", "ru", time.Second).
+		Transcribe(context.Background(), audio.Audio{Format: audio.PCM16kMono, Bytes: []byte("x")})
+	if !errors.Is(err, ErrInvalidTranscript) {
+		t.Fatalf("error = %v, want ErrInvalidTranscript", err)
+	}
+}
+
 func TestHTTPTranscriberRefusesWrongFormat(t *testing.T) {
 	t.Parallel()
 	a := audio.Audio{Format: audio.Format{SampleRate: 44100, Channels: 2, SampleBits: 16, Encoding: "pcm_s16le"}}