fix(recursion): set is_sub_builder flag in new_sub_builder

[MozirDmitriy]

Motivation

The is_sub_builder flag was never being set to true when creating sub-builders via new_sub_builder(). This meant all the assert guards in witness_var(), witness_felt(), witness_ext(), and commit_public_value() were effectively dead code - they could never trigger.

This is problematic because witness_* functions use local counters (witness_var_count, etc.) that start at 0 in each sub-builder,
creating index conflicts if called from within loops or conditionals.

Solution

• Set is_sub_builder = true in new_sub_builder() so the protection actually works
• Remove the assert from commit_public_value() since it's safely called from sub-builders inside commit_public_values() (the nb_public_values variable is properly shared between builders)

PR Checklist

• Added Tests
• Added Documentation
• Breaking changes