fix(internal/librarian/dart): add Protobuf specification check (#3965)

Add check toModelConfig function to ensure that the specification format
is set to "protobuf" for dart. This will prevent unsupported formats
like OpenAPI from being processed by the Dart generator.

---------

Signed-off-by: James Wu <jameslynnwu@users.noreply.github.com>

Author: jameslynnwu

internal/librarian/dart/codec.go

@@ -15,6 +15,8 @@
 package dart
 
 import (
+	"errors"
+	"fmt"
 	"strings"
 
 	"github.com/googleapis/librarian/internal/config"
@@ -23,7 +25,13 @@ import (
 	"github.com/googleapis/librarian/internal/sidekick/source"
 )
 
+var errInvalidSpecificationFormat = errors.New("dart generation requires protobuf specification format")
+
 func toModelConfig(library *config.Library, ch *config.API, sources *source.Sources) (*parser.ModelConfig, error) {
+	if library.SpecificationFormat != "" && library.SpecificationFormat != config.SpecProtobuf {
+		return nil, fmt.Errorf("%w, got %q", errInvalidSpecificationFormat, library.SpecificationFormat)
+	}
+
 	src := addLibraryRoots(library, sources)
 
 	if library.DescriptionOverride != "" {

internal/librarian/dart/codec_test.go

@@ -405,6 +405,17 @@ func TestToModelConfig(t *testing.T) {
 				},
 			},
 		},
+		{
+			name: "unsupported specification format",
+			library: &config.Library{
+				SpecificationFormat: "openapi",
+			},
+			channel: &config.API{
+				Path: "google/api/apikeys/v2",
+			},
+			googleapisDir: googleapisDir,
+			wantErr:       errInvalidSpecificationFormat,
+		},
 	} {
 		t.Run(test.name, func(t *testing.T) {
 			sources := &source.Sources{
@@ -413,10 +424,14 @@ func TestToModelConfig(t *testing.T) {
 			got, err := toModelConfig(test.library, test.channel, sources)
 			if test.wantErr != nil {
 				if !errors.Is(err, test.wantErr) {
-					t.Errorf("toModelConfig() error = %v, wantErr %v", err, test.wantErr)
+					t.Errorf("toModelConfig() error: %v, wantErr: %v", err, test.wantErr)
 				}
 				return
 			}
+			if err != nil {
+				t.Errorf("toModelConfig() unexpected error: %v", err)
+				return
+			}
 			if diff := cmp.Diff(test.want, got); diff != "" {
 				t.Errorf("mismatch (-want +got):\n%s", diff)
 			}