refactor(functions): use string arrays instead of Sets in kit install helpers (#11008)

### Description
Refactors function kit installation helper APIs (`extractExistingFunctionsInfo`, `promptKitInstanceId`, `promptKitId`, `generateUniqueId`, and `ExistingFunctionsInfo`) to use standard `string[]` arrays instead of `Set<string>` per codebase conventions (using intrinsic arrays in module APIs).

### Scenarios Tested
- Unit tests in `src/functions/kits/install.spec.ts` (`npm run mocha:fast -- src/functions/kits/install.spec.ts`).
- Verified unique ID generation and suffix collision avoidance with string arrays.
- Verified existing kit, codebase, and instance ID validation and prompt collisions.

### Sample Commands
N/A (Internal refactoring)