test: add utf8 string operation tests to highlight substr inconsistencies

[f4t4nt]

The current implementation of UTF-8 substr operations behaves differently from standard frameworks like pandas/polars/spark. This PR adds tests to document these inconsistencies, which will need to be fixed to ensure compatibility.

The test files:

• tests/table/utf8/test_substr.py: Documents current behavior and expected behavior
• tests/table/utf8/test_concat.py: Related UTF-8 string operation tests
• tests/table/utf8/test_length.py: Related UTF-8 string operation tests

Note: test_substr_baseline.py is a temporary file used to demonstrate the behavior in other frameworks and should be removed once the implementation is fixed.

[codspeed-hq]

CodSpeed Performance Report

Merging #3699 will degrade performances by 16.95%

_{Comparing nishant-utf8-substr-framework-parity (a973300) with main (5549d16)}

Summary

⚡ 1 improvements
❌ 1 regressions
✅ 25 untouched benchmarks

⚠️ Please fix the performance issues or acknowledge them on CodSpeed.

Benchmarks breakdown

	Benchmark	main	nishant-utf8-substr-framework-parity	Change
❌	test_iter_rows_first_row[100 Small Files]	198 ms	238.4 ms	-16.95%
⚡	test_show[100 Small Files]	23.8 ms	16.4 ms	+45.16%