feat(transcribe): the gateway sends language=sv, omits it for auto, and refuses anything else (#61)

The request used to hard-code language=sv for every upload, so a recording in another
language was decoded as Swedish. The language now comes from the context; no language in
the context is still sv.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

internal/adapters/transcribe/gateway.go

@@ -71,9 +71,20 @@ func (g *Gateway) Transcribe(ctx context.Context, path string) (domain.Transcrip
 		return domain.Transcription{}, fmt.Errorf("transcribe: stat audio: %w", err)
 	}
 
+	// The language is one request field and a closed set: sv is sent, auto sends nothing so
+	// the engine detects it, and anything else never leaves this process (tapir#61).
+	fields := [][2]string{{"model", g.model}, {"response_format", "verbose_json"}}
+	switch lang := domain.LanguageFrom(ctx); lang {
+	case domain.LangSwedish:
+		fields = append(fields, [2]string{"language", string(lang)})
+	case domain.LangAuto:
+	default:
+		return domain.Transcription{}, fmt.Errorf("transcribe: language %q is not one tapir sends", lang)
+	}
+
 	var prefix bytes.Buffer
 	mw := multipart.NewWriter(&prefix)
-	for _, kv := range [][2]string{{"model", g.model}, {"response_format", "verbose_json"}, {"language", "sv"}} {
+	for _, kv := range fields {
 		if err := mw.WriteField(kv[0], kv[1]); err != nil {
 			return domain.Transcription{}, fmt.Errorf("transcribe: build request: %w", err)
 		}

internal/adapters/transcribe/gateway_test.go

@@ -255,3 +255,48 @@ func TestGateway_AnErrorBodyIsBoundedToTwoHundredBytesInTheStatusError(t *testin
 	require.ErrorAs(t, err, &se)
 	assert.LessOrEqual(t, len(se.Snippet), 200, "a gateway's whole body must not become part of an error")
 }
+
+// The language travels on the context (tapir#61); the request field is the only place it
+// takes effect, so these observe the request the gateway fake received.
+
+func TestGateway_SendsLanguageSvWhenTheContextCarriesNoneAndForSwedish(t *testing.T) {
+	for name, ctx := range map[string]context.Context{
+		"no language in the context": context.Background(),
+		"swedish":                    domain.WithLanguage(context.Background(), domain.LangSwedish),
+	} {
+		t.Run(name, func(t *testing.T) {
+			var got seen
+			srv := serve(t, 200, verboseJSON, &got)
+			g := transcribe.NewGateway(srv.URL+"/v1", "k", "iguana/kb-whisper", srv.Client())
+
+			_, err := g.Transcribe(ctx, wavFile(t, "RIFF-synthetic-pcm"))
+
+			require.NoError(t, err)
+			assert.Equal(t, "sv", got.fields["language"])
+		})
+	}
+}
+
+func TestGateway_OmitsTheLanguageFieldForAnAutoContextSoTheEngineDetectsIt(t *testing.T) {
+	var got seen
+	srv := serve(t, 200, verboseJSON, &got)
+	g := transcribe.NewGateway(srv.URL+"/v1", "k", "iguana/kb-whisper", srv.Client())
+
+	_, err := g.Transcribe(domain.WithLanguage(context.Background(), domain.LangAuto), wavFile(t, "RIFF-synthetic-pcm"))
+
+	require.NoError(t, err)
+	_, present := got.fields["language"]
+	assert.False(t, present, "an auto upload must not be forced to Swedish")
+	assert.Equal(t, "iguana/kb-whisper", got.fields["model"], "the rest of the request is unchanged")
+}
+
+func TestGateway_ALanguageOutsideTheClosedSetIsRefusedBeforeAnythingIsSent(t *testing.T) {
+	var got seen
+	srv := serve(t, 200, verboseJSON, &got)
+	g := transcribe.NewGateway(srv.URL+"/v1", "k", "iguana/kb-whisper", srv.Client())
+
+	_, err := g.Transcribe(domain.WithLanguage(context.Background(), domain.Language("en")), wavFile(t, "RIFF-synthetic-pcm"))
+
+	require.Error(t, err)
+	assert.Empty(t, got.method, "no request may reach the engine with a language the closed set does not hold")
+}