feat(domain): Language, a closed set carried to the engines on the context (#61)

sv and auto; an empty form value is sv, anything else is refused. LanguageFrom defaults to
sv so a caller that sets nothing behaves as before.

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

internal/domain/language_test.go

@@ -0,0 +1,41 @@
+package domain_test
+
+import (
+	"context"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+
+	"git.d-ma.be/mathias/tapir/internal/domain"
+)
+
+func TestParseLanguage_KnownValuesAndEmptyAreAcceptedEverythingElseIsNot(t *testing.T) {
+	tests := []struct {
+		in     string
+		want   domain.Language
+		wantOK bool
+	}{
+		{"sv", domain.LangSwedish, true},
+		{"auto", domain.LangAuto, true},
+		{"", domain.LangSwedish, true}, // a client that sends no field keeps today's behaviour
+		{"SV", "", false},              // exact match: the value reaches a request field
+		{" sv", "", false},
+		{"sv\n", "", false},
+		{"en", "", false}, // a specific other language is not offered yet
+		{"sv,en", "", false},
+		{"../sv", "", false},
+	}
+	for _, tt := range tests {
+		t.Run(tt.in, func(t *testing.T) {
+			got, ok := domain.ParseLanguage(tt.in)
+			assert.Equal(t, tt.wantOK, ok)
+			assert.Equal(t, tt.want, got)
+		})
+	}
+}
+
+func TestLanguageFrom_IsSwedishWhenTheContextCarriesNoneAndOtherwiseWhatWasSet(t *testing.T) {
+	assert.Equal(t, domain.LangSwedish, domain.LanguageFrom(context.Background()))
+	ctx := domain.WithLanguage(context.Background(), domain.LangAuto)
+	assert.Equal(t, domain.LangAuto, domain.LanguageFrom(ctx))
+}