chore(librariangen): update snippet metadata in both directories

[JoeWang1127]

Updates the UpdateSnippetsMetadata logic in the legacylibrarian Go container to support and update snippet metadata files in both:

• the shared directory (internal/generated/snippets/<moduleName>)
• the module's examples directory (<moduleName>/examples)

Fixes googleapis/librarian#6257

[gemini-code-assist]

Code Review

This pull request updates the snippet metadata processing to support reading and updating metadata files from two distinct directories: the shared snippets directory and the examples directory. While the added unit tests successfully cover these new scenarios, the current implementation introduces non-determinism by iterating over a map and silently ignores cases where neither file is found. It is recommended to refactor the logic using a slice of paths to ensure deterministic processing, reduce duplication, and preserve the original logging behavior when files are missing.