Allow PlaceholderConfig.Text to accept either a single string or an
array of strings, from which one is randomly selected at runtime.
This maintains backward compatibility with existing single-string configs
while enabling random placeholder selection.
Changes:
- Modify PlaceholderConfig.Text type from string to FlexibleStringSlice
- Add GetRandomText() helper method for random selection
- Update SendPlaceholder in all channels to use GetRandomText()
- Update config.example.json with array placeholder examples
- Update Matrix channel documentation
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