Creates a new address pool for monitoring addresses.

Business Rules:

• Pool name must be unique within your organization.
• risk_configs is required (1..3 entries): each entry binds one risk config to one risk level. Levels must be unique within the pool, a config may be bound only once, and LOW cannot be bound (it is the no-risk floor).
• open_case_risk_levels is optional; every entry must be one of the levels mapped in risk_configs. A clean-to-risky flip at one of these levels opens a case.
• Config-to-level bindings are immutable after creation. Name and open_case_risk_levels can be changed via UpdatePool.