MAINT: Split TAP node execution scheduling

[Roman Lutz (romanlutz)]

Description

TreeOfAttacksWithPruningAttack previously owned bounded node dispatch alongside tree orchestration. This change extracts that mechanical scheduling responsibility into a focused internal _TreeOfAttacksNodeExecutor, colocated in tree_of_attacks.py so the TAP implementation remains easy to navigate.

Nodes continue to own branch-local state and per-node conversation/scoring behavior. The attack continues to own branching, scorer-driven pruning, visualization, persistence references, best-node tracking, and result construction. Batch order, concurrency, failure propagation, cancellation behavior, TAP/PAIR APIs, and identifiers remain unchanged.

Tests and Documentation

• Added deterministic coverage for batch ordering, node-state isolation, failure behavior, and cancellation propagation.
• Ran the TAP/PAIR unit selection: 124 passed.
• Ruff, ty, and commit hooks passed for the changed files.
• Documentation was not changed because this is an internal, behavior-preserving responsibility split.