fix(expr): support multibyte Unicode for overlay and ascii

[xiangjinwu]

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What's changed and what's your intention?

Fixes #9118

Checklist For Contributors

• I have written necessary rustdoc comments
• I have added necessary unit tests and integration tests
• All checks passed in ./risedev check (or alias, ./risedev c)

Documentation

Types of user-facing changes

• SQL commands, functions, and operators

Release note

Support multibyte Unicode for overlay and ascii.

[codecov]

Codecov Report

Merging #9321 (24dc46b) into main (8052dec) will increase coverage by 0.00%.
The diff coverage is 84.61%.

@@           Coverage Diff           @@
##             main    #9321   +/-   ##
=======================================
  Coverage   70.81%   70.81%           
=======================================
  Files        1219     1219           
  Lines      202512   202519    +7     
=======================================
+ Hits       143400   143416   +16     
+ Misses      59112    59103    -9     

Flag	Coverage Δ
rust	70.81% <84.61%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/expr/src/vector_op/overlay.rs	93.84% <83.33%> (-6.16%)	⬇️
src/expr/src/vector_op/ascii.rs	100.00% <100.00%> (ø)

... and 3 files with indirect coverage changes

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[BugenZhao]

LGTM