BUG: Fix Dataset.argmax warning and error messages

[nightcityblade]

Description

Fix the Dataset.argmax() deprecation warning and invalid-dimension error messages, which incorrectly referred to argmin because of a copy-paste mistake. Add focused regression coverage for both messages.

This addresses one small-cleanup item in #11721; it does not close the full tracking issue.

Checklist

• Closes #xxxx — N/A; Tracking: open deprecations #11721 tracks multiple independent deprecation cleanups.
• Tests added
• User visible changes (including notable bug fixes) are documented in whats-new.rst
• New functions/methods are listed in api.rst — N/A; no API was added.

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: OpenAI Codex
    Prompt: Fix the issue-backed Dataset.argmax() copy-paste message bug, add focused tests and a release note, and run relevant tests and lint.

[headtr1ck]

That test is not really useful, you can just drop it.

[nightcityblade]

Removed the message-specific test as requested and kept the PR focused on the implementation and release note. I amended the existing commit (5853eb7). Validation: the focused Dataset reduction tests pass (2 passed), and Ruff check/format plus git diff --check pass.

[nightcityblade]

CI follow-up: the minimum-versions job's only failure is an unrelated Zarr/Blosc decompression error in TestZarrRegionAuto::test_zarr_region_chunk_partial_offset[2] after 22,061 tests passed. The same matrix job passed on the prior revision. I attempted to rerun the failed job, but GitHub restricts reruns here to repository admins; a maintainer rerun should confirm it is transient.

[headtr1ck]

You can ignore the error, it's fixed in #11727

[headtr1ck]

Thx!