Eliminates placeholder substitution (${KEY}) in favor of storing complete
connection strings as single encrypted values. SecureStore now auto-creates
entries for all connection strings defined in appsettings. ConfigManager
editor reads/writes values directly to SecureStore.
Enable ConfigManager to validate runtime configuration (SecureStore secrets,
connection strings, LDAP) using the same validators as the Host application.
Adds AddInfrastructureValidators() extension for shared validator registration.