Revert "Reapply "buildconfig: disable tests for undecoded keys for now""

This reverts commit f2c16ad.

Author: mvo5

bib/internal/buildconfig/config.go

@@ -62,11 +62,15 @@ func decodeTomlBuildConfig(r io.Reader, what string) (*externalBlueprint.Bluepri
 	dec := toml.NewDecoder(r)
 
 	var conf externalBlueprint.Blueprint
-	_, err := dec.Decode(&conf)
+	metadata, err := dec.Decode(&conf)
 	if err != nil {
 		return nil, fmt.Errorf("cannot decode %q: %w", what, err)
 	}
 
+	if len(metadata.Undecoded()) > 0 {
+		return nil, fmt.Errorf("cannot decode %q: unknown keys found: %v", what, metadata.Undecoded())
+	}
+
 	return &conf, nil
 }
 

bib/internal/buildconfig/config_test.go

@@ -134,6 +134,16 @@ func TestReadLegacyJSONConfig(t *testing.T) {
 	assert.Equal(t, expectedBuildConfig, cfg)
 }
 
+func TestTomlUnknownKeysError(t *testing.T) {
+	fakeUserCnfPath := makeFakeConfig(t, "config.toml", `
+[[birds]]
+name = "toucan"
+`)
+	_, err := buildconfig.ReadWithFallback(fakeUserCnfPath)
+
+	assert.ErrorContains(t, err, "unknown keys found: [birds birds.name]")
+}
+
 func TestJsonUnknownKeysError(t *testing.T) {
 	fakeUserCnfPath := makeFakeConfig(t, "config.json", `
 {