Chore | Refactor to use unstructured.Unstructured instead of raw YAML strings

[dag-andersen]

Summary

• GetManifests and AppsetGenerate now return []unstructured.Unstructured instead of raw YAML strings
• YAML/JSON parsing moved into the CLI and API operation layers instead of happening later in the pipeline
• Replaced string-based ReplaceAll for app ID substitution with a recursive object traversal (replaceAppIdInManifests)
• Moved and renamed processYamlOutput → parseYAMLManifests (now in pkg/argocd/call_cli.go)
• Added replaceAppIdInManifests utility with tests in pkg/extract/utils.go