fix(ai): save all modified model defaults together

[Cypher-Aura-19]

Summary

• preserve pending model selections while switching between AI default scopes
• save all modified defaults with one action
• reconcile complete and partial save results correctly
• prevent duplicate operations and state updates after unmount
• add regression coverage for multi-scope saving

Root cause

Each scope stored its pending selection inside ScopeDetail, which was remounted whenever the selected scope changed. This discarded unsaved selections.

The save handler also accepted and persisted only the currently selected scope, so there was no way to submit changes across Site, Content, Data, and Plugins together.

Changes

• lift pending selections to DefaultsTab
• calculate dirty scopes against confirmed server values
• submit only modified scopes using one Save action
• keep failed selections pending when a partial save fails
• mark successful scopes clean immediately
• ensure retries submit only the scopes that previously failed
• disable editing, clearing, and saving while an operation is active
• guard asynchronous state updates after unmount
• keep clearing isolated to the selected scope
• show pending state and the number of defaults being saved

Testing

• bun test src/admin/pages/ai/tabs/DefaultsTab.test.tsx
  • 12 passed
  • 0 failed
• bun run lint
  • passed
• bun run build
  • passed
• git diff --check
  • passed

The full test suite has 207 failures on this branch. A clean upstream/main baseline produced the same 207 failures:

• branch-only failures: 0
• upstream-only failures: 0

No new full-suite failures were introduced by this change.

Checklist

• Tests cover the changed behaviour
• Multiple scopes can be edited before saving
• Only modified scopes are submitted
• Partial failures retain retryable changes
• Existing clear behaviour remains supported
• Lint and production build pass
• No database or backend changes
• No new dependencies
• No unrelated files included

Closes #290