Accept stacked PR base branches in analyze-pr (#59109)

Summary:
The `analyze_pr` check fails every PR in a stack above the first (example: #58867), since each targets the branch of the PR below it rather than `main` or a `-stable` branch.

- Pass the branch target check when the base branch is the head of another open PR in the repo.
- Other base branches still fail as before, and the parent PR's own check still guards its target.

NOTE: The workflow runs on `pull_request_target` from `main`, so this takes effect only after it lands.

Changelog: [Internal]

Pull Request resolved: #59109

Test Plan:
```
yarn jest .github/workflow-scripts/__tests__/checkBranchTarget-test.js
```

Checked the lookup the workflow makes against GitHub:

```
gh api 'repos/react/react-native/pulls?state=open&head=react:pr58950'
```

✅ Returns #58950, so #58951 (based on `pr58950`) will pass.
ℹ️ A non-PR branch returns no results and still fails.

Reviewed By: christophpurrer

Differential Revision: D125867260

Pulled By: Abbondanzo

fbshipit-source-id: e30b4768525f5c4938e782231b9b9fc435953f42

Author: huntie

.github/workflow-scripts/__tests__/checkBranchTarget-test.js

@@ -0,0 +1,47 @@
+/**
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ *
+ * @format
+ */
+
+'use strict';
+
+const checkBranchTarget = require('../checkBranchTarget');
+
+describe('checkBranchTarget', () => {
+  test('passes for main', () => {
+    expect(checkBranchTarget('main')).toEqual({
+      message: '',
+      status: 'PASS',
+      shouldAddPickLabel: false,
+    });
+  });
+
+  test('passes for a stable branch and adds the pick label', () => {
+    expect(checkBranchTarget('0.76-stable')).toEqual({
+      message: '',
+      status: 'PASS',
+      shouldAddPickLabel: true,
+    });
+  });
+
+  test('passes for the head branch of another open PR', () => {
+    expect(checkBranchTarget('pr58950', true)).toEqual({
+      message: '',
+      status: 'PASS',
+      shouldAddPickLabel: false,
+    });
+  });
+
+  test('fails for any other branch', () => {
+    const {message, status, shouldAddPickLabel} =
+      checkBranchTarget('feature-branch');
+
+    expect(status).toBe('FAIL');
+    expect(message).toContain('`feature-branch`');
+    expect(shouldAddPickLabel).toBe(false);
+  });
+});

.github/workflow-scripts/checkBranchTarget.js

@@ -13,15 +13,17 @@
  * Validates that the PR targets an appropriate base branch.
  *
  * @param {string} baseRef - The base branch ref (e.g., 'main', '0.76-stable')
+ * @param {boolean} [isStacked] - Whether the base branch is the head of another
+ *   open PR, as in a stack of PRs
  * @returns {{message: string, status: 'PASS'|'FAIL', shouldAddPickLabel: boolean}}
  */
-function checkBranchTarget(baseRef) {
+function checkBranchTarget(baseRef, isStacked = false) {
   const isMain = baseRef === 'main';
   const isStable = baseRef.endsWith('-stable');
 
   let message = '';
   let status = 'PASS';
-  if (!isMain && !isStable) {
+  if (!isMain && !isStable && !isStacked) {
     status = 'FAIL';
     message = `> [!CAUTION]
 > **Invalid Base Branch**

.github/workflows/analyze-pr.yml

@@ -44,8 +44,15 @@ jobs:
               repo: context.repo.repo,
               pull_number: context.issue.number,
             });
+            const {data: basePullRequests} = await github.rest.pulls.list({
+              owner: context.repo.owner,
+              repo: context.repo.repo,
+              state: 'open',
+              head: `${context.repo.owner}:${pullRequest.base.ref}`,
+            });
             const {message, status, shouldAddPickLabel} = checkBranchTarget(
               pullRequest.base.ref,
+              basePullRequests.length > 0,
             );
 
             if (shouldAddPickLabel) {