Fix diff-js-api-changes to compare PR head vs merge base (#55245)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/55245

Changelog: [INTERNAL] [FIXED] - Fix diff-js-api-changes workflow to correctly compare PR head vs merge base

The `diff-js-api-changes` action was comparing main to main instead of comparing the PR head to the point of main it branched from.

The workflow now:
1. Checks out main in `danger-pr.yml` to get the trusted scripts
2. Fetches the PR head commit and computes the merge base (the point it branched from main)
3. Extracts the API snapshots from both refs using `git show` to read-only temp files
4. Runs main's diff script to compare the two snapshots

**Security notes:**
- `git fetch` only downloads git objects, it does not modify the working directory
- `git show <sha>:path` extracts a file as read-only data, not executable code
- All executed scripts come from main (trusted), PR content is only used as data
- The PR's `.d.ts` file is written to a temp directory and passed as input to main's diff script

Reviewed By: huntie

Differential Revision: D90978905

fbshipit-source-id: fc9b420a27c84f1812b436f41d3169fad4f91291