test: Enhance tests for RLE and PCO casting: add assertions for dtype…

[SYaoJun]

… preservation and validity after nullability changes

Summary

Two tests were only asserting on the output dtype after a cast operation,
without verifying that the actual values were correctly preserved. This
means the tests would pass even if the cast silently corrupted data.

Closes: #000

Testing

tests passed

[connortsui20]

Thanks for the PR!

[codspeed-hq]

Merging this PR will improve performance by 12.36%

⚡ 2 improved benchmarks
✅ 952 untouched benchmarks
⏩ 1466 skipped benchmarks¹

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
⚡	Simulation	map_each[BufferMut<i32>, 128]	858.1 ns	770.6 ns	+11.36%
⚡	Simulation	bitwise_not_vortex_buffer_mut[128]	530.3 ns	471.9 ns	+12.36%

---

_{Comparing SYaoJun:0301_test (e31b284) with develop (fb71117)}

Footnotes

1. 1466 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

[connortsui20]

@SYaoJun Could you fix the lint? Then we should be good to go. Don't worry about the perf regression, it's just wrong :/

[SYaoJun]

@SYaoJun Could you fix the lint? Then we should be good to go. Don't worry about the perf regression, it's just wrong :/

I have fixed the lint.