fix: favor endpoint entry when one exists (#157)

Fixes FIPS endpoints for global services like IAM where the region is specified as `iam-fips`.

Author: aajtodd

client-runtime/aws-client-rt/common/src/aws/sdk/kotlin/runtime/endpoint/internal/Partition.kt

@@ -108,10 +108,10 @@ internal fun Partition.resolveEndpoint(region: String): Endpoint? {
 }
 
 private fun Partition.endpointDefinitionForRegion(region: String): EndpointDefinition {
-    val match = if (!isRegionalized) {
-        endpoints[partitionEndpoint]
-    } else {
-        endpoints[region]
+    val match = when {
+        endpoints.containsKey(region) -> endpoints[region]
+        !isRegionalized -> endpoints[partitionEndpoint]
+        else -> null
     }
 
     // return a matching definition or create an empty one that will be used for

client-runtime/aws-client-rt/common/test/aws/sdk/kotlin/runtime/endpoint/internal/EndpointsTest.kt

@@ -48,6 +48,10 @@ val testPartitions = listOf(
             "partition" to EndpointDefinition(
                 hostname = "some-global-thing.amazonaws.cn",
                 credentialScope = CredentialScope(region = "cn-east-1")
+            ),
+            "fips-partition" to EndpointDefinition(
+                hostname = "some-global-thing-fips.amazonaws.cn",
+                credentialScope = CredentialScope(region = "cn-east-1")
             )
         )
     ),
@@ -109,7 +113,27 @@ class EndpointsTest {
                 signingRegion = "eu-west-1",
                 signingName = "foo"
             )
-        )
+        ),
+        ResolveTest(
+            description = "specified partition endpoint",
+            region = "partition",
+            Endpoint(
+                hostname = "some-global-thing.amazonaws.cn",
+                protocol = "https",
+                signingRegion = "cn-east-1",
+                signingName = "foo"
+            )
+        ),
+        ResolveTest(
+            description = "fips partition endpoint",
+            region = "fips-partition",
+            Endpoint(
+                hostname = "some-global-thing-fips.amazonaws.cn",
+                protocol = "https",
+                signingRegion = "cn-east-1",
+                signingName = "foo"
+            )
+        ),
     )
 
     @Test