fix: out of text range while composing error

[kyungilcho]

Problem:

Screen_Recording_20241202-230013.mp4

Re-positioning the cursor in the middle of sentence while composing occurs following error.

so I add logic for checking validity of cursor position.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 72.25%. Comparing base (817c965) to head (c8b245b).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #979   +/-   ##
=======================================
  Coverage   72.25%   72.25%           
=======================================
  Files         318      318           
  Lines       15132    15134    +2     
=======================================
+ Hits        10933    10935    +2     
  Misses       4199     4199           

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[LucasXu0]

@kyungilcho Your PR may fix the composing error, but I think the attached position is still incorrect. The position is empty if the assertion happens.