fix(integrations): prevent duplicate workspace creation on oauth resubmit

[sung17]

Summary

• The shared OAuth callback (Messenger/Instagram/TikTok/Zalo/...) creates a new workspace whenever state carries no workspaceId, with no protection against the same OAuth attempt hitting the callback twice (slow provider round-trip + an impatient re-submit). Each hit created its own "New Workspace" and consumed a "workspaces" quota unit — only the first ever got a channel attached, leaving the rest as empty, quota-consuming orphans (visibly for TikTok/Zalo via a "duplicate" error, silently for Messenger/Instagram via pending-auth cookie overwrite).
• Add createWorkspaceForNewOauthState (apps/builder/src/lib/oauth-workspace.ts): locks and caches the created workspace by a hash of state + userId for 600s, so a repeat hit from the same user reuses the workspace instead of creating another one. The lock and the cached value use separate Redis keys — sharing one key would let the cache write clobber the lock's own release bookkeeping. Falls back to creating fresh if a cached id points at a workspace deleted in the interim.
• Fix workspace-switcher.tsx using workspace.name (not workspace.id) as a list key — duplicate "New Workspace" names (the default name for every fresh OAuth-created workspace) were triggering React's duplicate-key warning.

Changes

• apps/builder/src/app/integrations/[...integration]/callback.ts — delegate new-workspace creation on the shared OAuth callback to the deduping helper.
• apps/builder/src/lib/oauth-workspace.ts (new) — lock + cache the created workspace per state + userId, with a graceful fallback if the cached workspace was deleted.
• apps/builder/src/components/workspace-switcher.tsx — key list items by workspace.id instead of workspace.name.

Test plan

• pnpm lint
• pnpm --filter builder check-types
• Manual verification: trigger the same "connect new workspace via TikTok" OAuth flow twice in a row (simulating a slow-response resubmit) and confirm only one workspace + one quota unit is created, with the second hit reusing the same workspace.
• Manual verification: two different user accounts connecting a brand-new workspace around the same time do NOT collide onto the same workspace.