feat(ime): implement on_confirm_preedit handler

[kkimdev]

Split from #30 for easier review.

The on_confirm_preedit handler was a no-op — when the host IME confirmed a
preedit string, the guest was never notified and the preedit text was lost.

Fix

Cache the preedit text from on_preedit_string in TextInputState.current_preedit,
then on confirm_preedit flush it to the guest as a v3 commit_string:

1. Send v3 preedit_string("") to clear the underline
2. Send v3 commit_string with the cached text
3. Send v3 done to apply

If current_preedit is empty, only done is sent (no-op confirmation).

Changes

• on_confirm_preedit: commit cached preedit via v3 preedit_string + commit_string + done
• New field: TextInputState.current_preedit: String
• 3 tests: confirm with text, confirm empty, unknown host