Disable VCS triggers and add Slack notification test build

Temporarily disable VCS triggers to save TeamCity credits:
- GitHub PR checks
- TestingBuildType (all test configs)
- PropertyBased, RandomOrderTests, PluginVerifier, Nvim

Add SlackNotificationTest build to test changelog generation
and Slack notification flow separately.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>