fix: handle multibyte characters in physical plan tree renderer

[rustyconover]

Which issue does this PR close?

• Closes Tree-format physical plan renderer can panic on multibyte UTF-8 text #24663.

Rationale for this change

Rendering a physical plan in tree format can panic when an operator name or detail contains multibyte UTF-8 characters. The renderer measures text in characters but previously used those character positions as byte offsets when slicing strings. Those offsets are not necessarily valid UTF-8 boundaries.

What changes are included in this PR?

Use character-based iteration consistently when truncating and wrapping text in the physical plan tree renderer. Also use the number of characters, rather than the number of bytes, when checking for remaining text.

Are these changes tested?

Yes. Unit tests cover both wrapping and truncating text containing multibyte characters.

The following checks pass:

• cargo fmt --all -- --check
• cargo test -p datafusion-physical-plan --lib
• cargo clippy -p datafusion-physical-plan --all-targets --all-features -- -D warnings

Are there any user-facing changes?

EXPLAIN output using the tree renderer no longer panics when rendered plan details contain multibyte UTF-8 characters. There are no public API changes.

[saadtajwar]

Gotta love Rust 😆 this makes sense & LGTM!

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.44%. Comparing base (63f5b55) to head (c3dda53).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #24664   +/-   ##
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  Coverage   81.44%   81.44%           
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  Files        1118     1118           
  Lines      399550   399568   +18     
  Branches   399550   399568   +18     
=======================================
+ Hits       325398   325414   +16     
+ Misses      55154    55153    -1     
- Partials    18998    19001    +3     

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[nuno-faria]

Thanks @rustyconover. Could you also add a sqllogictest at explain_tree.slt? Something like this which currently panics on main:

create table t (a varchar);
explain select * from t where a = '🌤🌤🌤🌤🌤🌤🌤🌤🌤🌤🌤🌤🌤🌤🌤🌤🌤🌤🌤🌤🌤🌤🌤🌤';