fix(sql): recognize PGUSERNAME environment variable

Add PGUSERNAME to the PostgreSQL username resolution logic, matching
the documented behavior. The precedence order is now: options.username,
options.user, PGUSERNAME, PG_USER, PGUSER, USER, then "postgres".

Fixes #26684

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

src/js/internal/sql/shared.ts

@@ -692,7 +692,8 @@ function parseOptions(
       break;
     }
     case "postgres": {
-      username ||= options.username || options.user || env.PG_USER || env.PGUSER || env.USER || "postgres";
+      username ||=
+        options.username || options.user || env.PGUSERNAME || env.PG_USER || env.PGUSER || env.USER || "postgres";
       break;
     }
   }

test/js/sql/adapter-env-var-precedence.test.ts

@@ -25,7 +25,7 @@ describe("SQL adapter environment variable precedence", () => {
     'MARIADB_URL', 'MARIADBURL',
     'TLS_MARIADB_DATABASE_URL',
     'SQLITE_URL', 'SQLITEURL',
-    'PGHOST', 'PGUSER', 'PGPASSWORD', 'PGDATABASE', 'PGPORT',
+    'PGHOST', 'PGUSER', 'PGUSERNAME', 'PGPASSWORD', 'PGDATABASE', 'PGPORT',
     'MYSQL_HOST', 'MYSQL_USER', 'MYSQL_PASSWORD', 'MYSQL_DATABASE', 'MYSQL_PORT'
   ];
 
@@ -142,6 +142,30 @@ describe("SQL adapter environment variable precedence", () => {
     expect(options.options.username).toBe("postgres-user");
   });
 
+  test("PGUSERNAME should take precedence over PGUSER (issue #26684)", () => {
+    process.env.USER = "generic-user";
+    process.env.PGUSER = "pguser-fallback";
+    process.env.PGUSERNAME = "pgusername-primary";
+
+    const options = new SQL({
+      adapter: "postgres",
+    });
+
+    expect(options.options.username).toBe("pgusername-primary");
+  });
+
+  test("PGUSER should work when PGUSERNAME is not set", () => {
+    process.env.USER = "generic-user";
+    process.env.PGUSER = "pguser-value";
+    // PGUSERNAME is not set
+
+    const options = new SQL({
+      adapter: "postgres",
+    });
+
+    expect(options.options.username).toBe("pguser-value");
+  });
+
   test("should infer mysql adapter from MYSQL_URL env var", () => {
     process.env.MYSQL_URL = "mysql://user:pass@host:3306/db";