Per-provider naming constraints for ReqManagedAgents.Provisioner.Name.

max_length and charset differ sharply between providers: AgentCore's harnessName is capped at 40 characters over [a-zA-Z][a-zA-Z0-9_], while the Claude Managed Agents agent name allows 256 with no documented charset restriction. Parameterising both keeps one fit-and-fallback implementation without pushing AWS's charset onto names that never needed it — forcing [a-zA-Z0-9_] onto CMA would rewrite every hyphenated agent name and rotate resources for no benefit.

The two constructors below are the only supported policies. The struct itself is public, though, so a hand-built policy whose max_length cannot hold a digest plus the truncation hash is possible — ReqManagedAgents.Provisioner.Name.compose/3 rejects one with a message naming the minimum rather than failing obscurely inside the fit.