[kumo] Correct Toast manager docs and return types

[ftzi]

Issue

• packages/kumo/ai/USAGE.md and Toasty JSDoc use Toast.useToastManager().notify(), but the manager exposes add().
• Generated declarations for wrapped Toast managers return any for add, update, and promise, losing Base UI's return types and promise callback inference.

Solution

• Docs: Replace the stale notify() examples with add().
• Signatures: Add explicit Base UI-compatible generic signatures so add returns string, update returns void, and promise preserves Promise<Value> and callback inference.
• Regression coverage: Add focused runtime and type tests for the wrapped return values and promise inference.
• Preserved behavior: Keep existing forwarding and duplicate-ID handling unchanged.
• Changeset: Add a patch changeset for @cloudflare/kumo.

Validation

• pnpm run format:check, pnpm run lint, pnpm run typecheck, and pnpm run ci:typecheck
• pnpm run test (52 files, 1,276 tests) and pnpm run test:ci (9 tests)
• clean pnpm --filter @cloudflare/kumo build with declaration and registry generation, attw, and publint; generated-declaration consumer typecheck; changeset validation
• No browser testing was performed; the patch does not change rendered behavior.

AI disclosure

This change and pull request description were prepared by an AI coding agent at the contributor's direction.

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• Reviews
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• Tests
  • Tests included/updated
  • Automated tests not possible - manual testing has been completed as follows:
  • Additional testing not necessary because: