fix(download_lt): emit a warning when no checksum available

[mdevolde]

fix(download_lt): emit a warning when no checksum available

Why the pull request was made

To ensure that users are warned when they download a LanguageTool zip for which there isn't any available checksum.

Summary of changes

• Add a warn in language_tool_python.download_lt._get_zip_hash when no checksum is found.
• Add language_tool_python.download_lt._external_stacklevel to adapt stacklevel of warnings in language_tool_python.download_lt._get_zip_hash, according to which class the user is using.
• Adapted doc and tests.

Screenshots (if appropriate):

Not applicable.

How has this been tested?

Applied local tests, looked at the warning issued.

Resources

Not applicable.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update (changes to documentation only)
• Refactor / code style update (non-breaking change that improves code structure or readability)
• Tests / CI improvement (adding or updating tests or CI configuration only)
• Chore / maintenance (non-breaking change that does not affect functionality, such as updating dependencies or fixing typos)
• Other (please describe):

Checklist

• Followed the project's contributing guidelines.
• Updated any relevant tests.
• Updated any relevant documentation.
• Added comments to your code where necessary.
• Formatted your code, run the linters, checked types and tests.
• Added your changes to the CHANGELOG file, if applicable.

[Copilot]

Pull request overview

This PR improves the UX and safety signaling of LanguageTool ZIP downloads by warning users when an integrity checksum is unavailable, and adjusts warning attribution so it points to the external caller rather than internal library frames.

Changes:

• Emit a RuntimeWarning from _get_zip_hash() when no SHA-256 checksum is available for the requested LanguageTool version.
• Add _external_stacklevel() to dynamically compute warnings.warn(..., stacklevel=...) based on the call chain.
• Update docs/tests and add a changelog entry for the new warning behavior.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 4 comments.

File	Description
tests/unit/test_download_unit.py	Adds/updates unit tests to assert warning behavior for missing vs known checksums.
src/language_tool_python/download_lt.py	Implements missing-checksum warning and dynamic warning stacklevel computation.
docs/source/references/env_vars.rst	Documents that missing checksums now produce a RuntimeWarning and how to configure per-version checksums.
CHANGELOG.md	Records the user-visible warning behavior change for downloads without checksums.

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (75c7d1e) to head (3a0ba5e).
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Additional details and impacted files

@@            Coverage Diff            @@
##            master      #216   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files           13        13           
  Lines         1696      1713   +17     
=========================================
+ Hits          1696      1713   +17     

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