refactor: clean up config retrieval

Author: marc0der

src/main/kotlin/io/sdkman/broker/config/AppConfig.kt

@@ -48,12 +48,11 @@ class DefaultAppConfig : AppConfig {
 
     // Persistence backend selector
     override val persistenceBackend: PersistenceBackend =
-        config.getStringOrDefault("persistence.backend", PersistenceBackend.Mongo.configValue).let { raw ->
-            PersistenceBackend.fromConfigValue(raw).getOrElse {
+        config.getOptionString("persistence.backend")
+            .flatMap { PersistenceBackend.fromConfigValue(it) }
+            .getOrElse {
                 throw IllegalArgumentException(
-                    "Invalid PERSISTENCE_BACKEND value '$raw'. " +
-                        "Supported values: ${PersistenceBackend.supportedValues.joinToString(", ")}."
+                    "PERSISTENCE_BACKEND value not found or incorrect. Supported values: mongo, postgres."
                 )
             }
-        }
 }

src/main/kotlin/io/sdkman/broker/config/PersistenceBackend.kt

@@ -1,7 +1,7 @@
 package io.sdkman.broker.config
 
 import arrow.core.Option
-import arrow.core.toOption
+import arrow.core.getOrNone
 
 enum class PersistenceBackend(
     val configValue: String
@@ -13,8 +13,6 @@ enum class PersistenceBackend(
         private val configValueToBackend: Map<String, PersistenceBackend> =
             entries.associateBy { it.configValue }
 
-        fun fromConfigValue(value: String): Option<PersistenceBackend> = configValueToBackend[value].toOption()
-
-        val supportedValues: List<String> = entries.map { it.configValue }
+        fun fromConfigValue(value: String): Option<PersistenceBackend> = configValueToBackend.getOrNone(value)
     }
 }

src/test/kotlin/io/sdkman/broker/config/AppConfigSpec.kt

@@ -126,8 +126,7 @@ class AppConfigSpec :
 
             // when/then: construction must blow up so the broker never serves traffic against an unsupported backend
             val error = shouldThrow<IllegalArgumentException> { DefaultAppConfig() }
-            error.message shouldContain "cassandra"
-            error.message shouldContain "mongo"
-            error.message shouldContain "postgres"
+            error.message shouldContain "PERSISTENCE_BACKEND value not found or incorrect."
+            error.message shouldContain "Supported values: mongo, postgres."
         }
     })