fix: handle ZodEnum in cloneStringType and add type safety checks

- cloneStringType now accepts both ZodString and ZodEnum (for prompts
  that use raw schema.shape without toPermissiveShape)
- Add runtime type checks that throw clear errors for unexpected types
- Add 2 unit tests for ZodEnum handling in addCompletions

Agent-Logs-Url: https://github.com/advanced-security/codeql-development-mcp-server/sessions/e316ece4-06c5-45d7-8020-062e6ec67e39

Co-authored-by: data-douser <70299490+data-douser@users.noreply.github.com>

Author: Copilot