feat(checks): report uninteded list conversion in rst

[nijel]

Starting translation with a list marker will make docutils treat it as list what will either look weird or break the build.

[Copilot]

Pull request overview

This PR adds detection for unintended list conversion in reStructuredText translations. When a translation starts with a list marker (like * or -) but the source text doesn't, it can cause the text to be rendered as a list item, which may break the document structure or render incorrectly.

• Adds a new check in RSTSyntaxCheck to detect when translations inappropriately start with list markers
• Includes comprehensive test coverage for the new list marker detection
• Fixes an existing issue in Japanese documentation where a list marker was incorrectly added

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
weblate/checks/markup.py	Adds list marker detection logic to RSTSyntaxCheck.check_single() and changes return type of validate_rst_snippet() from list to tuple for better caching
weblate/checks/tests/test_markup_checks.py	Adds test_list() method with multiple test cases covering both valid and invalid list marker scenarios
docs/locales/ja/LC_MESSAGES/docs.po	Fixes a real-world example by removing an incorrectly added list marker from the Japanese translation
docs/changes.rst	Documents the new feature in the changelog for version 5.15.2