update error message, add test for resolution order

Author: xinsong-cui

runtime/auth/http-auth/common/src/aws/smithy/kotlin/runtime/http/auth/EnvironmentBearerTokenProvider.kt

@@ -28,7 +28,7 @@ public class EnvironmentBearerTokenProvider(
     override suspend fun resolve(attributes: Attributes): BearerToken {
         val bearerToken = sysPropKey?.let { platform.getProperty(it) }
             ?: envKey?.let { platform.getenv(it) }
-            ?: error("environment variable is not set")
+            ?: error("neither system property $sysPropKey nor environment variable $envKey is set")
         return object : BearerToken {
             override val token: String = bearerToken
             override val attributes: Attributes = mutableAttributes().apply {

runtime/auth/http-auth/common/test/aws/smithy/kotlin/runtime/http/auth/EnvironmentBearerTokenProviderTest.kt

@@ -37,6 +37,22 @@ class EnvironmentBearerTokenProviderTest {
         assertEquals("test-sys-props-bearer-token", token.token)
     }
 
+    @Test
+    fun testResolutionOrder() = runTest {
+        val provider = EnvironmentBearerTokenProvider(
+            "TEST_SYS_PROPS_TOKEN",
+            "TEST_ENV_TOKEN",
+            TestPlatformProvider(
+                props = mutableMapOf("TEST_SYS_PROPS_TOKEN" to "test-sys-props-bearer-token"),
+                env = mutableMapOf("TEST_ENV_TOKEN" to "test-env-bearer-token"),
+            ),
+        )
+
+        val token = provider.resolve()
+
+        assertEquals("test-sys-props-bearer-token", token.token)
+    }
+
     @Test
     fun testResolveWithMissingToken() = runTest {
         val provider = EnvironmentBearerTokenProvider(
@@ -51,6 +67,6 @@ class EnvironmentBearerTokenProviderTest {
         val exception = assertFailsWith<IllegalStateException> {
             provider.resolve()
         }
-        assertEquals("environment variable is not set", exception.message)
+        assertEquals("neither system property MISSING_TEST_TOKEN nor environment variable MISSING_TEST_TOKEN is set", exception.message)
     }
 }