fix(ime): only send set_surrounding_text to host when dirty

[kkimdev]

Split from #30 for easier review.

on_commit used state.surrounding_text.take() to consume the value, so if
on_commit was called twice without an intervening on_set_surrounding_text
(e.g. content_type and cursor_rect updates), the second commit would send
stale or empty surrounding text.

Fix

Replace .take() with a dedicated surrounding_text_dirty flag. The flag is
set when on_set_surrounding_text is received and cleared after forwarding
once.

Changes

• New: TextInputState.surrounding_text_dirty: bool
• on_set_surrounding_text: set dirty flag instead of relying on .take()
• on_commit: check dirty flag, clear after send
• 1 test verifying dirty/re-send semantics