Move SubplanSpawnError from local subplan_service definition to centralized
cleveragents.core.exceptions alongside four new spec-defined error types:
SubplanExecutionError, MaxParallelExceededError, and SubplanDepthLimitError.
Per the v3.3.0 specification (AUTO-ARCH-6), all subplan-related errors are
defined in exceptions.py with proper inheritance hierarchy under DomainError/
PlanError/BusinessRuleViolation. The old local SpawnValidationError class has
been replaced with SubplanSpawnError(PlanError) with a simplified constructor
API (message string instead of validation_errors list).
Updates:
- exceptions.py: Added 4 subplan error classes + __all__ entries
- subplan_service.py: Remove local SpawnValidationError, import SubplanSpawnError from exceptions
- services/__init__.py: Update TYPE_CHECKING stub and _LAZY_IMPORTS for new location
- vulture_whitelist.py: Replace old entry with new error class names
- docs/reference/subplan_service.md: Update to reference SubplanSpawnError (v3.3.0)
- features/*.feature + steps: Update test references from SpawnValidationError to SubplanSpawnError
HAL9000
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