System allow to load IDT mixed bases with index out of range declaration

[AlexeyGirin]

Steps to Reproduce

1. Toggle to Macro - Flex mode
2. Load IDT from paste from clipboard way: (B5:00653005)
   

Actual behavior
System loads nucleoside successfully

Expected behavior
System throws an error - Index out of range

As per requirements:

Custom mixed bases are represented in ... (Ni:XXYYZZQQ), where i=1..4,

Versions

• Ketcher Version 2.25.0-rc.1 Build at 2024-09-03; 15:31:03
• Indigo Toolkit Version 1.24.0-rc.1.0-gb8f099851-wasm32-wasm-clang-19.0.0
• Chrome Version 128.0.6613.114 (Official Build) (64-bit)
• Win10

Reference issues: #1619