fix(python): translate Polars fill_null with expression fills

[danking]

Translate Polars fill_null to native Vortex fill_null when the fill argument is a literal. For expression fills, such as another column, use zip_(is_null(child), fill, child) because native fill_null currently requires a constant fill value. Both arguments are recursively translated; no eager evaluation is performed.

The tests compare the same predicate over Polars-only and Vortex-backed LazyFrames and check explicit expected row IDs. Three literal-fill cases additionally assert that translation emits native fill_null; the column-fill case covers a null replacement value.

Validation: the column-fill case previously passed with the mapping and raised Polars ComputeError wrapping NotImplementedError when the converter change was reverted. The latest native literal-fill change, its three new regression cases, and lint have not been run.

[codspeed]

Merging this PR will regress 2 benchmarks

⚠️ Unknown Walltime execution environment detected

Using the Walltime instrument on standard Hosted Runners will lead to inconsistent data.

For the most accurate results, we recommend using CodSpeed Macro Runners: bare-metal machines fine-tuned for performance measurement consistency.

⚠️ 1 benchmark measured no execution time

Nothing ran under measurement, usually because the compiler removed the code under test. This result is not comparable, so it counts as unchanged.

Preventing compiler optimizations

⚠️ Different runtime environments detected

Some benchmarks with significant performance changes were compared across different runtime environments,
which may affect the accuracy of the results.

Open the report in CodSpeed to investigate

⚡ 2 improved benchmarks
❌ 2 regressed benchmarks
✅ 2171 untouched benchmarks
🆕 2 new benchmarks
⏩ 409 skipped benchmarks¹

Warning

Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
❌	Simulation	sum_v2_i64	193.8 µs	224.7 µs	-13.73%
❌	Simulation	sum_i64	193.9 µs	224.6 µs	-13.67%
⚡	Simulation	decode_primitives[i64, (1000, 32)]	37.7 µs	24.6 µs	+53.59%
⚡	Simulation	random_i128[0.01]	8.1 µs	7.2 µs	+12.68%
🆕	Simulation	varbinview_fill_null_inlined	N/A	276.4 µs	N/A
🆕	Simulation	varbinview_fill_null_outlined	N/A	288.6 µs	N/A
⚠️	Simulation	bench_compare_primitive[(10000, 2)]	< 1 ns	< 1 ns	N/A

Tip

Investigate this regression by commenting @codspeedbot fix this regression on this PR, or directly use the CodSpeed MCP with your agent.

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_{Comparing dk/polars-fill-null (babf64d) with develop (53c7659)²}

Footnotes

1. 409 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. ↩

2. No successful run was found on develop (30e84dc) during the generation of this report, so 53c7659 was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩