The previous validation allowed "b1" which is invalid because 'b' prefix
expects 2-digit integers (10-99), not 1-digit. This caused ValidationError
when selecting old format content.
Now validates that:
- 'a' prefix: 1 digit (a0-a9)
- 'b' prefix: 2 digits (b10-b99)
- 'c' prefix: 3 digits (c100-c999)
- etc.
Invalid indices are converted to 'a1' as a safe default.
🤖 Generated with [Claude Code](https://claude.com/claude-code)
Co-Authored-By: Claude <noreply@anthropic.com>
