tests: add actionable error test for mismatched closing tag in markup

[lesliehc]

Type of changes

• Bug fix
• New feature
• Documentation / docstrings
• Tests
• Other

Checklist

• I've run the latest black with default args on new code. (poetry run black --check . passed)
• I've updated CHANGELOG.md and CONTRIBUTORS.md where appropriate (N/A for tests-only; maintainers may advise)
• I've added tests for new code (this PR adds a single test)
• I accept that @willmcgugan may be pedantic in the code review.

Description

Add a focused unit test that asserts malformed markup with a mismatched closing tag
(e.g., "[bold]hello[/italic]") raises a MarkupError and that the message is
actionable. The assertions are intentionally flexible to avoid over-constraining the
exact wording while still guarding for helpful terms such as “invalid / mismatch /
closing / end tag”.

This is tests-only and does not change runtime behavior. It helps prevent future
regressions in error messaging clarity for invalid markup.

Relates to: #2993.