feat: replace Redpanda Admin API with Kafka SCRAM API for user management

Replace all Redpanda Admin API calls in the user service (v1, v1alpha2,
and REST handlers) with Kafka SCRAM protocol operations
(DescribeUserSCRAMCredentials / AlterUserSCRAMCredentials). This removes
the dependency on the Redpanda Admin API for user management, making the
user service work across Redpanda and non-Redpanda Kafka clusters.

- Add DescribeUserSCRAMCredentials and AlterUserSCRAMs to console servicer
- Add NewConnectErrorFromKafkaError helper for Kafka error translation
- ListUsers now uses DescribeUserSCRAMCredentials and returns mechanism
- CreateUser/UpdateUser use AlterUserSCRAMs with UpsertSCRAM
- DeleteUser describes credentials first, then deletes all mechanisms
- REST handlers no longer gate behind RedpandaClientProvider
- Remove HasRedpandaAPI from user endpoint compatibility checks
- Update proto annotations from API_REDPANDA_ADMIN to API_KAFKA
- Add unit tests using kfake and update integration tests

Author: weeco

backend/pkg/api/connect/errors/kafka.go

@@ -21,6 +21,20 @@ import (
 	v1alpha2 "github.com/redpanda-data/console/backend/pkg/protogen/redpanda/api/dataplane/v1alpha2"
 )
 
+// NewConnectErrorFromKafkaError creates a connect.Error from a Kafka error.
+// If the error is a *kerr.Error it delegates to NewConnectErrorFromKafkaErrorCode;
+// otherwise it wraps the error as CodeInternal.
+func NewConnectErrorFromKafkaError(err error) *connect.Error {
+	if kafkaErr, ok := errors.AsType[*kerr.Error](err); ok {
+		return NewConnectErrorFromKafkaErrorCode(kafkaErr.Code, nil)
+	}
+	return NewConnectError(
+		connect.CodeInternal,
+		err,
+		NewErrorInfo(v1alpha2.Reason_REASON_KAFKA_API_ERROR.String()),
+	)
+}
+
 // NewConnectErrorFromKafkaErrorCode creates a new connect.Error for a given Kafka error code.
 // Kafka error codes are described in the franz-go kerr package.
 func NewConnectErrorFromKafkaErrorCode(code int16, msg *string) *connect.Error {
@@ -78,8 +92,7 @@ func KeyValsFromKafkaError(err error) []KeyVal {
 		return []KeyVal{}
 	}
 
-	var kafkaErr *kerr.Error
-	if errors.As(err, &kafkaErr) {
+	if kafkaErr, ok := errors.AsType[*kerr.Error](err); ok {
 		return []KeyVal{
 			{
 				Key:   "kafka_error_code",