resample: preserve integer precision in nodata mask comparison (#2570)

[brendancol]

Closes #2570.

Summary

• _resolve_nodata no longer routes integer sentinels through float(); it casts to the input dtype instead so the equality test in _apply_nodata_mask runs in integer space.
• For int64 sentinels above 2**53 (e.g. nodata = 2**60 + 1), valid cells that share the rounded float representation (e.g. 2**60) are no longer masked out.
• Float inputs still route through float() so the existing NaN short-circuit keeps working.

Backend coverage

The fix lives in the shared masking helper that every backend goes through (numpy / cupy / dask+numpy / dask+cupy). Verified on numpy and dask+numpy locally.

Test plan

• New TestNodataIntegerPrecision class in xrspatial/tests/test_resample.py covering:
  • int64 sentinel above 2**53 via explicit kwarg
  • int64 sentinel via _FillValue attr
  • int32 sentinel (negative min)
  • uint16 sentinel (max)
  • float NaN sentinel short-circuit still works
  • float finite sentinel still works
• Pre-existing TestNodata cases still pass
• Full test_resample.py suite (184 tests) green
• test_resample_coverage_2026_05_27.py and test_resample_signature_annot_2544.py green

[brendancol]

PR Review: resample integer nodata precision fix

Blockers (must fix before merge)

None.

Suggestions (should fix, not blocking)

• xrspatial/resample.py:1162-1165 -- the try/except TypeError around the NaN check is dead code. isinstance(nodata, float) already gates the path, and np.isnan on a Python float will never raise TypeError. Drop the try wrapper and keep the if isinstance(...) and np.isnan(...) check on its own line.

Nits (optional improvements)

• xrspatial/resample.py:1166 -- np.asarray(nodata).astype(agg.dtype).item() will silently truncate a non-integer float sentinel (e.g. nodata=-9999.5 on int32 data becomes -9999). Realistically nobody passes a fractional sentinel, but a one-line check like if isinstance(nodata, float) and not nodata.is_integer(): raise ValueError(...) would catch the obvious user error. Skip if you'd rather keep the helper minimal.
• xrspatial/tests/test_resample.py:1456 -- test_int64_sentinel_above_2pow53_preserves_distinct_values is the headline regression test. Consider asserting out.values[1, 1] == np.float32(1 << 60) so a future regression that masks the cell to NaN AND a regression that swaps the surviving value both get caught. The current np.isfinite check is correct but a bit loose.

What looks good

• The split between float-input and integer-input paths in _resolve_nodata is the right call. It keeps the float NaN short-circuit working while routing integer comparisons through the input dtype.
• _apply_nodata_mask computes the mask before promoting to float32, which is exactly the surgical change the bug needs. The mask carries over correctly to agg.where(mask) after the cast.
• Backend coverage is right: the masking helper sits upstream of every per-backend dispatch path, so numpy, cupy, dask+numpy, and dask+cupy all inherit the fix automatically. The PR body confirms numpy and dask+numpy were exercised locally.
• Test coverage hits the exact failure mode (int64 sentinel above 2**53), the explicit-kwarg and attr paths, and the unaffected float branches.

Checklist

• Algorithm matches reference / intent (integer equality in input dtype)
• All implemented backends produce consistent results (single shared helper)
• NaN handling is correct (float NaN short-circuit preserved; NaN sentinel on int data is a no-op)
• Edge cases are covered by tests (int64 > 2**53, int32, uint16, float NaN, float finite)
• No dask chunk boundary concerns (no neighborhood op in this change)
• No premature materialization or unnecessary copies
• Benchmark: not applicable for a correctness fix in a helper
• README feature matrix: not applicable, no new function or backend
• Docstrings updated to reflect the new dispatch on input dtype

[brendancol]

Follow-up review

Re-ran the review after the address-nits commit (1adea6f). All previous findings are resolved:

• Suggestion (dead try/except): FIXED -- the wrapper is gone; the isinstance + np.isnan check stands alone.
• Nit (silent fractional truncation): FIXED -- _resolve_nodata now raises ValueError("nodata=... is not representable in integer dtype ...") when a non-integer float sentinel is passed for an integer input. Added test_fractional_float_sentinel_on_int_input_raises to lock it in.
• Nit (loose isfinite assertion): FIXED -- the int64-above-2**53 regression test now asserts the surviving cell equals np.float32(1 << 60), so masking-to-NaN and value-swap regressions both get caught.

Full test_resample.py suite passes locally (185 tests). No remaining blockers, suggestions, or nits.