Updates a pool's mutable properties: name and open-case risk levels.

Mutable fields (both optional — omitting a field leaves it unchanged):

• name: must be non-empty and unique within your organization.
• open_case_risk_levels: replaces the current set entirely. Every entry must name a level mapped by one of the pool's risk configs. An empty levels list clears the set.

Semantics: Changing open_case_risk_levels is forward-looking only. It never retro-opens cases for members already at an in-set level; the case gate fires on history rows entering the set (SCAN transition-in or ENROLL seed) after the change.

Immutable: Config-to-level bindings (risk_configs) cannot be changed after creation.

Authorization: Pool must belong to the requesting user's organization.