fix(Lists): improve nested list collapse for deep structures

[makhnatkin]

Fixes #1042

Improvements

• O(N) single-pass algorithm: replaced iterative O(K*N) approach with a single tree traversal that collects all redundant nesting in one pass
• Recursive flattening: handles deeply nested structures (up to 100 levels) with proper depth guards
• Correct position mapping: applies replacements in reverse order to preserve document positions, uses tr.mapping for accurate cursor placement

[gravity-ui]

[gravity-ui]