perf: Optimize lpad/rpad to remove unnecessary memory allocations per element

[andygrove]

Which issue does this PR close?

Closes #.

Rationale for this change

Benchmark	Before	After	Improvement
spark_lpad: default padding	441 µs	111 µs	-74.7%
spark_lpad: custom padding	437 µs	107 µs	-75.7%
spark_rpad: default padding	443 µs	108 µs	-75.4%
spark_rpad: custom padding	445 µs	107 µs	-75.8%
spark_lpad: multi-char padding	336 µs	158 µs	-52.3%
spark_rpad: multi-char padding	337 µs	145 µs	-56.1%
spark_lpad: with truncation	178 µs	82 µs	-54.7%
spark_rpad: with truncation	173 µs	79 µs	-54.0%

What changes are included in this PR?

1. Reusable buffer: Added a String buffer that is cleared and reused for each element instead of allocating a new String per element
2. Changed function signature: Replaced add_padding_string(String) -> Result with write_padded_string(&mut String, &str) that writes directly to a buffer
3. Pre-computed pad characters: The pad_string.chars() iterator is now collected once into a Vec before the loop
4. Direct padding writes: New write_padding_chars() function writes padding directly to the buffer without intermediate allocations, with a fast path for single-character padding
5. Eliminated unnecessary allocations: Removed string.parse().unwrap() which was converting &str to String unnecessarily

How are these changes tested?

[andygrove]

@coderfender fyi

[coderfender]

Thank you very much for the optimization @andygrove

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 59.62%. Comparing base (f09f8af) to head (a8e7848).
⚠️ Report is 797 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #2963      +/-   ##
============================================
+ Coverage     56.12%   59.62%   +3.49%     
- Complexity      976     1375     +399     
============================================
  Files           119      167      +48     
  Lines         11743    15488    +3745     
  Branches       2251     2567     +316     
============================================
+ Hits           6591     9234    +2643     
- Misses         4012     4956     +944     
- Partials       1140     1298     +158     

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[comphead]

👍

[comphead]

we can prob to the buffer fixed size?

String::with_capacity(rpad_length);

?

[andygrove]

Updated both of these. Thanks!

[comphead]

same?

[comphead]

Thanks @andygrove good benches