Fix padding an integer array with non-finite constant_values (NaN)

[PranavAchar01]

Description

Fixes padding an integer array with an explicit non-finite constant_values (e.g. np.nan), which previously raised ValueError: cannot convert float NaN to integer:

import numpy as np, xarray as xr
da = xr.DataArray(np.arange(9), dims="x")
da.pad({"x": (0, 1)}, "constant", constant_values=np.nan)   # used to raise

Variable.pad only promoted the dtype for the default fill sentinel (dtypes.NA), not for an explicit np.nan. It now promotes the dtype (via np.result_type) when an explicit non-finite fill value cannot be represented by an integer dtype, matching the promotion already done for the default fill value. Finite fill values keep numpy's existing casting behaviour (e.g. constant_values=1.23456 still truncates on an integer array).

Checklist

• Closes Bug when padding coordinates with NaNs #6431
• Tests added
• User visible changes (including notable bug fixes) are documented in whats-new.rst

AI Disclosure

• This PR contains AI-generated content.
  • I have tested any AI-generated content in my PR.
  • I take responsibility for any AI-generated content in my PR.
    Tools: Claude Code — prompted to locate and fix issue Bug when padding coordinates with NaNs #6431 with a localized change in Variable.pad plus a regression test. I have reviewed and tested the change locally (the pad test suites pass).

[Copilot]

Pull request overview

Fixes DataArray/Dataset/Variable.pad(..., mode="constant", constant_values=np.nan) on integer-backed arrays by promoting the target dtype so non-finite fill values (NaN/Inf) can be represented, instead of raising during padding.

Changes:

• Update Variable.pad to promote integer dtypes when an explicit non-finite constant_values is provided.
• Add regression tests covering explicit constant_values=np.nan and mixed tuple fills (e.g. (0, np.nan)).
• Document the behavior change in doc/whats-new.rst.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
xarray/core/variable.py	Adds dtype-promotion logic for explicit non-finite constant_values in constant padding mode.
xarray/tests/test_dataarray.py	Adds regression assertions for padding integer arrays with explicit NaN fill values.
doc/whats-new.rst	Notes the bug fix and dtype-promotion behavior for non-finite explicit constant_values.

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