docs(openfeature): document ESM-resolvable fallback entrypoint

Node's ESM resolver does not append `.js` to package subpaths without an exports map, so the extensionless fallback fails before file tracing starts. Pin both the ESM import and automatic nft discovery from the default entrypoint.

Author: BridgeAR

README.md

@@ -97,7 +97,7 @@ When using the experimental OpenFeature provider, file-traced deployments can fo
 its dependencies into the output with a side-effect import before accessing `tracer.openfeature`:
 
 ```js
-import 'dd-trace/openfeature'
+import 'dd-trace/openfeature.js'
 ```
 
 This is a fallback for build tools that do not recognize the provider's optional-require wrapper.

packages/dd-trace/test/openfeature/file-tracing.spec.js

@@ -1,6 +1,9 @@
 'use strict'
 
 const assert = require('node:assert/strict')
+const { spawnSync } = require('node:child_process')
+const { mkdirSync, mkdtempSync, rmSync, symlinkSync } = require('node:fs')
+const { tmpdir } = require('node:os')
 const path = require('node:path')
 
 const { nodeFileTrace } = require('@vercel/nft')
@@ -25,6 +28,10 @@ async function assertTracesProvider (entrypoint) {
 }
 
 describe('OpenFeature file tracing', () => {
+  it('traces the provider dependency tree through the default entrypoint', async () => {
+    await assertTracesProvider(path.join(repoRoot, 'index.js'))
+  })
+
   it('traces the provider dependency tree through the runtime wrapper', async () => {
     await assertTracesProvider(path.join(repoRoot, 'packages/dd-trace/src/openfeature/flagging_provider.js'))
   })
@@ -36,4 +43,23 @@ describe('OpenFeature file tracing', () => {
   it('loads the provider through the explicit entrypoint', () => {
     require(path.join(repoRoot, 'openfeature.js'))
   })
+
+  it('loads the explicit entrypoint as an ESM package subpath', () => {
+    const fixtureRoot = mkdtempSync(path.join(tmpdir(), 'dd-trace-openfeature-'))
+    const nodeModulesPath = path.join(fixtureRoot, 'node_modules')
+
+    try {
+      mkdirSync(nodeModulesPath)
+      symlinkSync(repoRoot, path.join(nodeModulesPath, 'dd-trace'), 'junction')
+      const result = spawnSync(
+        process.execPath,
+        ['--input-type=module', '--eval', "import 'dd-trace/openfeature.js'"],
+        { cwd: fixtureRoot, encoding: 'utf8' }
+      )
+
+      assert.strictEqual(result.status, 0, result.stderr)
+    } finally {
+      rmSync(fixtureRoot, { recursive: true, force: true })
+    }
+  })
 })