fix(server-runtime-injection): Load ESM hook without the build/esm package.json marker (#25562)

## What

The diagnostics-channel hook now loads as ESM even when
`build/esm/package.json` is missing from a deployment.

## Why

Vercel runs Node 22 with syntax detection off, so without that file the
hook failed to load and channel-based spans were lost.

Closes: #25480

---------

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

Author: andreiborza

packages/server-runtime-injection/src/register.ts

@@ -321,6 +321,25 @@ export function registerDiagnosticsChannelInjection(): void {
 
       const hookSpecifier = hookFound ? pathToFileURL(hookPath).href : '@sentry/server-runtime-injection/hook';
 
+      if (hookFound) {
+        // Only `build/esm/package.json` marks the hook and its chunks as ESM. Deployments can drop
+        // that file, and Vercel disables syntax detection, so Node then parses the hook as CommonJS
+        // and fails on its `export`s.
+        //
+        // This registers a small loader hook before the real one. Its `load` sets
+        // `format: 'module'` for every URL inside our ESM build directory and passes all other URLs
+        // through unchanged. Hooks registered earlier also apply when Node loads hook modules
+        // registered later, so the real hook and its chunks load as ESM even without the marker.
+        // The loader is inlined as a `data:` URL because Node always loads those as ESM, so it
+        // cannot fail the same way and needs no file of its own.
+        const esmDirUrl = `${pathToFileURL(dirname(hookPath)).href}/`;
+        mod.register(
+          `data:text/javascript,${encodeURIComponent(
+            `const d=${JSON.stringify(esmDirUrl)};export async function load(u,c,n){return n(u,u.startsWith(d)?{...c,format:'module'}:c)}`,
+          )}`,
+        );
+      }
+
       mod.register(hookSpecifier, {
         parentURL,
         data: { instrumentations: SENTRY_RUNTIME_INSTRUMENTATIONS, diagnosticsPort },

packages/server-runtime-injection/test/esmMarker.test.ts

@@ -0,0 +1,42 @@
+import { spawnSync } from 'node:child_process';
+import { cpSync, mkdtempSync, rmSync } from 'node:fs';
+import { join } from 'node:path';
+import { afterAll, describe, expect, it } from 'vitest';
+
+/**
+ * Deployments can drop `build/esm/package.json` (the only `"type": "module"` marker for the hook),
+ * and Vercel runs Node with syntax detection disabled, so the loader thread then parses the hook as
+ * CommonJS. Runs against the built files, so it needs the package built.
+ *
+ * The copy sits inside this package so `@sentry/*` still resolves from its `node_modules`.
+ */
+const copyDir = mkdtempSync(join(__dirname, '.esm-marker-'));
+cpSync(join(__dirname, '..', 'build'), join(copyDir, 'build'), { recursive: true });
+rmSync(join(copyDir, 'build', 'esm', 'package.json'));
+
+afterAll(() => {
+  rmSync(copyDir, { recursive: true, force: true });
+});
+
+describe('without the build/esm package.json marker', () => {
+  it('registers the ESM loader hook', { timeout: 30_000 }, () => {
+    // Removing `registerHooks` forces the `Module.register()` path, the one that loads the hook on
+    // Node < 24.13, whatever Node version runs the test.
+    const script = `
+      require('node:module').registerHooks = undefined;
+      const { registerDiagnosticsChannelInjection } = require(${JSON.stringify(join(copyDir, 'build', 'cjs', 'register.js'))});
+      registerDiagnosticsChannelInjection();
+      import('data:text/javascript,export default 1').then(() => {
+        const marker = globalThis.__SENTRY__?.__SENTRY_ORCHESTRION__ ?? globalThis.__SENTRY_ORCHESTRION__;
+        process.stdout.write(JSON.stringify(marker));
+      });
+    `;
+
+    const { stdout, stderr } = spawnSync(process.execPath, ['--no-experimental-detect-module', '-e', script], {
+      encoding: 'utf8',
+    });
+
+    expect(stderr).not.toContain('Failed to register diagnostics-channel injection hooks');
+    expect(JSON.parse(stdout)).toEqual({ runtime: [] });
+  });
+});