fix: Improve bullet-point resolution when plain text precedes bullet points

[vaind]

Summary

Fixes #120 where the updater script failed to detect bullet point formats when changelogs contained plain introductory text before any bullet points.

• Fixed bullet-point detection logic in update-changelog.ps1
• Added test case for changelogs with plain text before bullet points
• Improved string handling to prevent formatting issues

Changes

• updater/scripts/update-changelog.ps1: Fixed bullet-point detection logic on lines 182-184 to properly handle cases where no bullet points are found in the unreleased section
• updater/tests/update-changelog.Tests.ps1: Added specific unit test for plain text intro scenario
• updater/tests/testdata/changelog/plain-text-intro/: New test case files to verify the fix

Test plan

• All existing tests pass (6/6)
• New test case specifically validates bullet-point resolution with plain text
• Manual testing with the problematic CHANGELOG.md from the issue

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