r.fill.stats: migrate tests from gunittest to pytest

[Valyrian-Code]

Migrates the r.fill.stats testsuite from gunittest to pytest, continuing the one-at-a-time migrations from #7635/#7675/#7780/#7893.

Same input and reference outputs as before, now inlined and compared cell by cell with numpy instead of through a difference raster.

Found one discrepancy while doing that: the reference data has the northeast corner cell as null in all four modes, but the tool currently fills it in all four, with a different value per mode (as expected, since each mode computes a different statistic). Every other cell matches the old reference exactly, to float64 precision.

The old test's assertRastersNoDifference could not have caught this: it checks the range of a difference raster, and a difference against a null cell is itself null, so it gets excluded from that range check.

No relevant change to the fill algorithm since this test was added in 2013 explains the corner behavior, so I believe the original reference was wrong for that one cell. This test asserts the corner's current value explicitly rather than reusing the old expectation. Happy to open a separate issue if that seems worthwhile.

Verified locally, all four cases pass.

[Valyrian-Code]

Same CI infra issue as noted on #7780: the failing checks are hitting an HTTP 504 from Launchpad while adding the ubuntugis-unstable PPA, unrelated to this PR. Should clear with a re-run.

[Copilot]

Copilot review overview

🟡 Changes recommended

The NumPy comparison retains a relative tolerance, weakening the previous absolute 1e-6 assertion.

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Open (1)

• Set rtol=0 to preserve the previous absolute tolerance · New

What changed in this PR

Migrates r.fill.stats regression coverage from gunittest to pytest, using isolated GRASS sessions, grass.tools, and NumPy comparisons.

Changes:

• Adds parametrized tests for all four interpolation modes.
• Inlines input and reference raster data.
• Removes the legacy gunittest test and data files.

File	Description
raster/​r.fill.stats/​tests/​r_fill_stats_test.py	New pytest-based regression tests and inline fixtures.
raster/​r.fill.stats/​testsuite/​test_r_fill_stats.py	Removes the legacy gunittest test.
raster/​r.fill.stats/​testsuite/​data/​input_ascii.txt	Removes migrated input data.
raster/​r.fill.stats/​testsuite/​data/​output_wmean.txt	Removes migrated weighted-mean reference.
raster/​r.fill.stats/​testsuite/​data/​output_mean.txt	Removes migrated mean reference.
raster/​r.fill.stats/​testsuite/​data/​output_median.txt	Removes migrated median reference.
raster/​r.fill.stats/​testsuite/​data/​output_mode.txt	Removes migrated mode reference.

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[Valyrian-Code]

Good catch, fixed. The original was an absolute precision=1e-6, and the default rtol=1e-5 let a cell near 5 drift about 5e-5, so this had quietly loosened it. Set rtol=0, all four modes still pass.