Preserve named RSA dispatch through CryptoConfig

Replace resolved RSA.Create(string) with explicit CryptoConfig.CreateFromName dispatch and the same checked result type. Preserve configured mappings, null/unknown results, cast and constructor failures, and single name/construction evaluation. A pattern temporary avoids adding nullable cast warnings. Named/commented/unresolved calls, initializers, lambda/query contexts, other factories and conditional snapshots remain unchanged. Opt-in compatibility port; no algorithm or security policy is selected.