fix(devtools/cmd/migrate-sidekick): migrate to Library.Roots instead of RustCrate.Roots (#3332)

Migrate roots info into Library.Roots instead of Library.Rust.Roots. 

Migration script created a different struct as expected by
[toSidekickConfig](https://github.com/googleapis/librarian/blob/main/internal/librarian/internal/rust/codec.go#L25).
There is confusion that both of these Roots existed in config. Removed
Library.Rust.Roots as I expect this should not be Rust specific.

Expected showcase config after migration:
```
  - name: google-cloud-showcase-v1beta1
    version: 1.0.0
    channels:
      - path: schema/google/showcase/v1beta1
    copyright_year: "2025"
    output: src/generated/showcase
    roots:
      - showcase
      - googleapis
    rust:
      detailed_tracing_attributes: true
    skip_publish: true
```
Local run for both `google-cloud-showcase-v1beta1` and
`google-cloud-compute-v1` succeed after this change. (compute generate
produced diffs, filing different issue for it.)

Fix #3324, #3329

Author: zhumin8

devtools/cmd/migrate-sidekick/main.go

@@ -383,7 +383,9 @@ func buildGAPIC(files []string, repoPath string) (map[string]*config.Library, er
 		}
 
 		titleOverride, _ := sidekick.Source["title-override"].(string)
-		roots, _ := sidekick.Source["roots"].(string)
+		if roots, ok := sidekick.Source["roots"].(string); ok {
+			lib.Roots = strToSlice(roots, false)
+		}
 		includeList, _ := sidekick.Source["include-list"].(string)
 		includeIds, _ := sidekick.Source["include-ids"].(string)
 		skippedIds, _ := sidekick.Source["skipped-ids"].(string)
@@ -441,7 +443,6 @@ func buildGAPIC(files []string, repoPath string) (map[string]*config.Library, er
 			TitleOverride:             titleOverride,
 			PackageNameOverride:       packageNameOverride,
 			RootName:                  rootName,
-			Roots:                     strToSlice(roots, false),
 			DefaultFeatures:           strToSlice(defaultFeatures, false),
 			IncludeList:               strToSlice(includeList, false),
 			IncludedIds:               strToSlice(includeIds, false),

devtools/cmd/migrate-sidekick/main_test.go

@@ -186,6 +186,7 @@ func TestBuildGAPIC(t *testing.T) {
 					DescriptionOverride: "Description override",
 					SpecificationFormat: "discovery",
 					Output:              "testdata/read-sidekick-files/success-read/nested",
+					Roots:               []string{"discovery", "googleapis"},
 					Rust: &config.RustCrate{
 						RustDefault: config.RustDefault{
 							DisabledRustdocWarnings: []string{"bare_urls", "broken_intra_doc_links", "redundant_explicit_links"},
@@ -197,7 +198,6 @@ func TestBuildGAPIC(t *testing.T) {
 						TitleOverride:             "Google Apps Script Types",
 						PackageNameOverride:       "google-cloud-security-publicca-v1",
 						RootName:                  "conformance-root",
-						Roots:                     []string{"discovery", "googleapis"},
 						DefaultFeatures:           []string{"instances", "projects"},
 						IncludeList:               []string{"api.proto", "source_context.proto", "type.proto", "descriptor.proto"},
 						IncludedIds:               []string{".google.iam.v2.Resource"},

internal/config/language.go

@@ -133,9 +133,6 @@ type RustCrate struct {
 	// RootName is the root name for the crate.
 	RootName string `yaml:"root_name,omitempty"`
 
-	// Roots is a list of root names.
-	Roots []string `yaml:"roots,omitempty"`
-
 	// DefaultFeatures is a list of default features to enable.
 	DefaultFeatures []string `yaml:"default_features,omitempty"`