Fix release notes never being included in Slack notifications

[Copilot]

Description of Change

Release notes are always empty in Slack messages posted by publishApp. Two bugs in the changelog parsing pipeline:

1. VersionUtil.GetReleaseNotesFromChangelog uses line.Contains(changelogHeader) — literal string match against a regex pattern string. Never matches.
2. build.csx passes header pattern ## [[0-9]+.[0-9]+.[0-9]+] - [0-9]+-[0-9]+-[0-9]+ which includes a date suffix (- YYYY-MM-DD) that doesn't exist in the CHANGELOG (format is just ## [55.2.2]).

Fixes:

• VersionUtil.csx: line.Contains(changelogHeader) → Regex.IsMatch(line, changelogHeader)
• build.csx: Pattern changed to ## \[[0-9]+.[0-9]+.[0-9]+\] — matches actual CHANGELOG format, properly escapes brackets

Todos

• I have tested on an Android device.
• I have tested on an iOS device.
• I have supported accessibility

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[Copilot]

Pull request overview

This PR fixes a bug where release notes were always empty in Slack notifications posted by the publishApp build task. The issue was caused by two bugs in the changelog parsing pipeline: (1) using string literal matching instead of regex matching, and (2) using an incorrect header pattern that included a date suffix not present in the actual CHANGELOG.md format.

Changes:

• Fixed VersionUtil.GetReleaseNotesFromChangelog to use Regex.IsMatch instead of string.Contains for pattern matching
• Corrected the changelog header pattern in build.csx to match the actual format ## [X.Y.Z] with properly escaped brackets
• Updated CHANGELOG.md with patch version 55.2.3

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
build/build.csx	Fixed changelog header pattern from ## [[0-9]+.[0-9]+.[0-9]+] - [0-9]+-[0-9]+-[0-9]+ to ## \\[{VersionPattern}\\] to match actual CHANGELOG format
build/AwesomeBuildsystem/Core/VersionUtil.csx	Changed line.Contains(changelogHeader) to Regex.IsMatch(line, changelogHeader) to properly match regex patterns
CHANGELOG.md	Added entry for version 55.2.3 documenting this build fix