Add max_child_depth field to DecompositionConfig (default: 5, matching
plan.max-child-depth config) with validation in __post_init__. Update
_build_hierarchy() in DecompositionService to stop recursion and emit
a warning when the depth limit is reached.
BDD tests verify: max_child_depth guards trigger before max_depth
when more restrictive, warnings are logged, default config value is
correct, and invalid values are rejected.
ISSUES CLOSED: #10269
CoreRasurae