Deduplicate validation between AEES and StructuredEpisodeManager

[davidkfoss]

Problem

The same control_mode / context_mode / structured_control_mode / candidate-list invariants are validated in two places:

• scheduler.py:84-111 (AEES)
• episode.py:90-130 (StructuredEpisodeManager)

When a constraint changes, both must be updated or they drift.

Proposal

Pick one layer to own validation. The natural home is StructuredEpisodeManager (it owns the structured-control state); AEES should pass its config through and trust the manager to raise. Alternatively, extract a single _validate_control_config(...) helper used by both.

Acceptance criteria

• Only one place performs the invariant checks.
• The same exceptions (type + message) are still raised for the same bad inputs.
• Tests covering invalid-config paths still pass.