[umtong]

Description

Closes #6860.

align(join="override") currently takes coordinate metadata from the first input along with its index values. DataArrays lose their own attributes and encoding; Datasets can inherit additional entries from the first input. In a NetCDF round trip, inherited dtype="int16" and scale_factor=0.1 settings changed an otherwise unpacked coordinate from [1.04, 2.06] to [1.0, 2.1].

Keep the aligned coordinate values, but restore each receiving object's attributes and encoding from its matching index variable. This also preserves empty metadata mappings. The same round trip now retains [1.04, 2.06].

Expanded the existing alignment metadata test across DataArray and Dataset inputs, checking distinct and empty attributes/encoding on both inputs and results. Both override cases fail on the original code and pass with the fix.

Validation: the DataArray, Dataset and Coordinates suites pass (1,206 passed, 73 skipped, 5 xfailed, 2 xpassed). Direct checks also cover both copy modes, MultiIndex/custom indexes and NetCDF round trips. All applicable configured pre-commit hooks pass.

Checklist

• Closes Align with join='override' may update index coordinate metadata #6860
• User-visible fix 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: Codex desktop (gpt-6-astra, reasoning effort max) for implementation, regression tests, the release note, and this description.
Prompt (task summary): Fix #6860 so override alignment preserves each receiving coordinate's attributes and encoding, with regression coverage.

[Codex on behalf of @umtong]

Autonomous contribution by codex (GPT-6 Astra).