Document SQLGEN_TEST_PASSWORD in test helpers

Add doc comments explaining that SQLGEN_TEST_PASSWORD environment
variable can be set to override the default password for tests.

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

Author: mcraveiro

tests/mysql/test_helpers.hpp

@@ -6,6 +6,10 @@
 
 namespace sqlgen::mysql::test {
 
+/// Creates test credentials for MySQL.
+/// Set SQLGEN_TEST_PASSWORD environment variable to override the default
+/// password ("password"). This allows running tests against an existing
+/// database without modifying source code.
 inline Credentials make_credentials() {
   const char* env_password = std::getenv("SQLGEN_TEST_PASSWORD");
   return Credentials{.host = "localhost",

tests/postgres/test_helpers.hpp

@@ -6,8 +6,12 @@
 
 namespace sqlgen::postgres::test {
 
+/// Creates test credentials for PostgreSQL.
+/// Set SQLGEN_TEST_PASSWORD environment variable to override the default
+/// password ("password"). This allows running tests against an existing
+/// database without modifying source code.
 inline Credentials make_credentials() {
-  const char* env_password = std::getenv("PGPASSWORD");
+  const char* env_password = std::getenv("SQLGEN_TEST_PASSWORD");
   return Credentials{.user = "postgres",
                      .password = env_password ? env_password : "password",
                      .host = "localhost",