feat(storage): add GCS parity and public file URI semantics

[kkopanidis]

Summary

This expands the original GCS provider rewrite into a broader public-file URL contract for the storage module.

GCS provider parity:

• Implements the missing IStorageProvider methods for containers, folders, files, and signed URLs.
• Fixes exists so it returns the provider result instead of always returning true.
• Supports service account key file, inline JSON (google.serviceAccountKeyJson), and Application Default Credentials.
• Uses Uniform Bucket-Level Access and IAM (roles/storage.objectViewer for allUsers) for public buckets.

Public file URL semantics:

• Adds File.uri as a stable Conduit-relative path (/storage/getFileUrl/:id).
• Keeps File.url / sourceUrl as direct provider/CDN URLs only when both the container and file are public.
• Allows public files in private containers by storing uri only and signing provider URLs on demand through Conduit.
• Rejects private files in public containers, including omitted isPublic values.
• Removes unsupported 99/100-year signed URL branches from GCS, Azure, Aliyun, and AWS getPublicUrl handling.
• Adds a migration to backfill uri and clear stale direct URLs for public files in private containers.

Validation

• npm run build in modules/storage
• npm run build in libraries/grpc-sdk
• Focused ESLint on touched storage TypeScript files
• Composer 2.5 deslop pass
• Composer 2.5 verification pass

Manual test plan

• Configure storage provider google with a GCS bucket or emulator.
• Verify GCS auth via key file path, inline JSON, and ADC.
• Public container + public file stores sourceUrl, url, and uri.
• Public container + omitted/false isPublic throws a clear validation error.
• Private container + public file stores uri only; unauthenticated GET /storage/getFileUrl/:id returns a fresh signed URL.
• Private container + private file still requires auth and signs on demand.
• Folder/container/file lifecycle methods work for GCS, including exists returning false for missing files.
• Existing public files are migrated with uri, and private-container public files have stale direct URLs cleared.