Fix TestLoadMissingRequired to work in CI environment

CI sets DATABASE_URL environment variable, so the test was passing
when it should have failed. Now explicitly clearing all required
env vars with t.Setenv() which restores original values after test.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: cole-rgb

internal/config/config_test.go

@@ -30,8 +30,11 @@ func TestLoad(t *testing.T) {
 }
 
 func TestLoadMissingRequired(t *testing.T) {
-	// t.Setenv automatically clears after test
-	// Don't set required vars to test missing validation
+	// Clear required environment variables to test missing validation
+	// t.Setenv will restore the original values after the test
+	t.Setenv("DATABASE_URL", "")
+	t.Setenv("JWT_SECRET", "")
+	t.Setenv("S3_BUCKET", "")
 
 	_, err := Load()
 	if err == nil {