fix: preserve selected AskUserQuestion option

[kocaemre]

Summary

• Preserve the selected AskUserQuestion option when clients also return follow-up text
• Keep free-form-only answers working when no option is selected
• Add a regression test for the option + follow-up text case

Closes #1050

Test plan

• npm run test:run -- src/tests/elicitation.test.ts
• npm run build
• npm run check
• git diff --check

Full npm run test:run was also attempted; it reached 1042 passed / 27 skipped and failed only the existing root-environment resolvePermissionMode bypassPermissions expectations.