fix(telecom): use deterministic bill ID for state replay consistency

[CormickKneey]

Summary

Fix non-deterministic bill ID generation in telecom domain that causes state replay inconsistency during RL training.

Changes Made

• Replace random UUID-based bill ID generation (uuid.uuid4().hex[:8]) with deterministic ID based on customer_id and billing period ({customer_id}_{YYYYMM})
• Add test case to verify deterministic bill ID generation for state replay consistency

Testing

• Added test_apply_one_time_charge_creates_deterministic_bill_id test that verifies:
  • Two identical database states produce identical bill IDs when the same operation is applied
  • Bill IDs follow the new deterministic format
• All existing tests pass

Context

During RL training, the environment state needs to be replayed consistently. The original random UUID-based bill ID generation caused non-deterministic behavior - replaying the same actions on the same initial state produced different results due to random bill IDs. This made it impossible to properly validate trajectories during training.

Checklist

• Tests pass (make test)
• Code follows style guidelines (make check-all)
• Documentation updated
• Breaking changes noted