fix(sql): allow process to exit when PostgreSQL connection is idle

Previously, idle PostgreSQL connections would keep the Bun process alive
indefinitely after queries completed. This happened because `updateRef()`
always kept the poll reference when `pending_activity_count > 0`, which
includes any non-disconnected connection status.

This fix modifies `updateRef()` to unref the poll when the connection is
idle (connected with no pending queries and no data to write), matching
Node.js behavior where idle database connections allow the process to exit.

Fixes #3548

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

test/js/sql/sql.test.ts

@@ -3907,6 +3907,36 @@ CREATE TABLE ${table_name} (
       expect(result.stdout.toString().split("\n")).toEqual(["1", "2", ""]);
     });
 
+    test("idle connection allows process to exit #3548", async () => {
+      // This test verifies that idle PostgreSQL connections don't keep the process alive.
+      // Before the fix, the process would hang indefinitely after queries completed.
+      const file = path.posix.join(__dirname, "sql-fixture-unref.ts");
+
+      // Use Bun.spawn with a timeout to detect if the process hangs
+      await using proc = Bun.spawn([bunExe(), file], {
+        env: { ...bunEnv, DATABASE_URL: process.env.DATABASE_URL },
+        stdout: "pipe",
+        stderr: "pipe",
+      });
+
+      // Wait for exit with a 10 second timeout - process should exit much faster
+      const exitPromise = proc.exited;
+      const timeoutPromise = new Promise<"timeout">(resolve => setTimeout(() => resolve("timeout"), 10000));
+
+      const result = await Promise.race([exitPromise, timeoutPromise]);
+
+      if (result === "timeout") {
+        proc.kill();
+        throw new Error("Process hung - idle connection prevented exit (issue #3548)");
+      }
+
+      const stdout = await new Response(proc.stdout).text();
+      const stderr = await new Response(proc.stderr).text();
+
+      expect(stdout.trim()).toBe("query_done");
+      expect(result).toBe(0);
+    });
+
     describe("Boolean Array Type", () => {
       test("should handle empty boolean array", async () => {
         await using sql = postgres({ ...options, max: 1 });