chore: set release config as part of sidekick migrate (#3333)

Sidekick git commands expect a remote metadata for
[GetLastTag](https://github.com/googleapis/librarian/blob/f05b7bb29f6da7ecc4e791649c85c0b688888b07/internal/librarian/githelpers/githelpers.go#L44).
It looks like this should be populated by
[config.release](https://github.com/googleapis/librarian/blob/main/internal/librarian/publish.go#L69),
so adding to migration script.

For #3327

---------

Signed-off-by: ldetmer <1771267+ldetmer@users.noreply.github.com>
Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>

Author: ldetmer

devtools/cmd/migrate-sidekick/main.go

@@ -143,6 +143,11 @@ func run(args []string) error {
 
 	cfg := buildConfig(allLibraries, defaults)
 
+	cfg.Release = &config.Release{
+		Branch: "main",
+		Remote: "upstream",
+	}
+
 	if err := yaml.Write(*outputPath, cfg); err != nil {
 		return fmt.Errorf("failed to write config: %w", err)
 	}

devtools/cmd/migrate-sidekick/main_test.go

@@ -773,6 +773,8 @@ func TestRunMigrateCommand(t *testing.T) {
 					t.Logf("cleanup: remove %s: %v", outputPath, err)
 				}
 			})
+			wantReleaseBranch := "main"
+			wantReleaseRemote := "upstream"
 
 			if err := run([]string{test.path}); err != nil {
 				if test.wantErr == nil {
@@ -804,6 +806,15 @@ func TestRunMigrateCommand(t *testing.T) {
 						}
 					}
 				}
+				if librarianConfig.Release == nil {
+					t.Fatalf("expected Release config to be set")
+				}
+				if librarianConfig.Release.Branch != wantReleaseBranch {
+					t.Fatalf("want Release.Branch to be %s got %s", wantReleaseBranch, librarianConfig.Release.Branch)
+				}
+				if librarianConfig.Release.Remote != wantReleaseRemote {
+					t.Fatalf("want Release.Remote to be %s got %s", wantReleaseRemote, librarianConfig.Release.Remote)
+				}
 			}
 
 		})