refactor(test): use async spawn pattern in regression test

- Use `await using Bun.spawn()` instead of `Bun.spawnSync()`
- Remove forbidden negative assertions against stderr
- Keep positive assertions for stdout and exit code

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

src/bundler/bundle_v2.zig

@@ -689,7 +689,9 @@ pub const BundleV2 = struct {
         if (path.pretty.ptr == path.text.ptr) {
             // TODO: outbase
             const rel = bun.path.relativePlatform(transpiler.fs.top_level_dir, path.text, .loose, false);
-            path.pretty = bun.handleOom(this.allocator().dupe(u8, rel));
+            const pretty = bun.handleOom(this.allocator().dupe(u8, rel));
+            bun.path.platformToPosixInPlace(u8, pretty);
+            path.pretty = pretty;
         }
         path.assertPrettyIsValid();
 

test/regression/issue/026798.test.ts

@@ -0,0 +1,48 @@
+import { expect, test } from "bun:test";
+import { bunEnv, bunExe, tempDirWithFiles } from "harness";
+
+// Test for https://github.com/oven-sh/bun/issues/26798
+// Bun.build crashes on Windows when a plugin's onResolve returns null,
+// falling back to the standard resolver, and the resulting path.pretty
+// contains Windows backslashes.
+test("Bun.build with plugin onResolve returning null should not crash", async () => {
+  const dir = tempDirWithFiles("onresolve-null-test", {
+    "src/index.ts": `import { foo } from "./foo"; console.log(foo);`,
+    "src/foo.ts": `export const foo = "hello";`,
+    "build.ts": `
+const result = await Bun.build({
+  entrypoints: ["./src/index.ts"],
+  plugins: [
+    {
+      name: "null-returner",
+      setup(build) {
+        // Return null to trigger fallback to standard resolver
+        build.onResolve({ filter: /.*/ }, (args) => {
+          return null;
+        });
+      },
+    },
+  ],
+});
+
+if (!result.success) {
+  console.error("Build failed:", result.logs);
+  process.exit(1);
+}
+console.log("Build succeeded");
+`,
+  });
+
+  await using proc = Bun.spawn({
+    cmd: [bunExe(), "run", "build.ts"],
+    cwd: dir,
+    env: bunEnv,
+    stdout: "pipe",
+    stderr: "pipe",
+  });
+
+  const [stdout, exitCode] = await Promise.all([proc.stdout.text(), proc.exited]);
+
+  expect(stdout).toContain("Build succeeded");
+  expect(exitCode).toBe(0);
+});