test: Add comprehensive tests for Datadog API Key environment variable support

- Add testApiKeyFromConfigurationProperty: Verify API key resolution from config
- Add testApiKeyFromEnvironmentVariable: Test environment variable fallback
- Add testConfigurationPropertyTakesPrecedenceOverEnvironmentVariable: Ensure config takes priority
- Add testEmptyConfigurationPropertyFallsBackToEnvironmentVariable: Test empty config fallback
- Use reflection to test private getApiKey() method directly
- Avoid API validation issues in tests using environment variable manipulation

Author: barryib

flink-metrics/flink-metrics-datadog/src/test/java/org/apache/flink/metrics/datadog/DatadogHttpReporterFactoryTest.java

@@ -91,8 +91,7 @@ void testEmptyConfigurationPropertyFallsBackToEnvironmentVariable() throws Excep
             newEnv.put("DATADOG_API_KEY", envApiKey);
             CommonTestUtils.setEnv(newEnv, true);
 
-            final String resolvedApiKey =
-                    getApiKeyViaReflection(createConfig("apikey", ""));
+            final String resolvedApiKey = getApiKeyViaReflection(createConfig("apikey", ""));
             assertThat(resolvedApiKey).isEqualTo(envApiKey);
         } finally {
             CommonTestUtils.setEnv(originalEnv, true);