Replace branch check with head check in PlatformIO workflow

[JanPetterMG]

This pull request replaces the remote branch existence check with a direct GitHub API lookup for pull requests by head branch.

The previous implementation relied on git ls-remote to determine whether a branch existed. This caused unintended behavior where closing a PR and deleting its branch could result in a new PR being recreated during the next workflow run.

The updated logic checks whether a PR already exists (open or closed) for the generated head branch, ensuring fully idempotent behavior.

Key changes

• Removed git ls-remote branch existence checks.
• Added a github-script step that queries pull requests.
• Updated conditional workflow logic to skip PR creation when a PR for the head branch already exists.
• Ensured closed PRs prevent recreation of identical update PRs.
• Eliminated dependency between branch lifecycle and PR lifecycle.

Impact

• Prevents automatic recreation of PRs after manual closure and branch deletion.
• Makes dependency update automation fully idempotent.
• Improves correctness by operating at the PR abstraction level instead of raw git branches.
• Reduces unnecessary branch churn and duplicate PR noise.
• Scales efficiently by querying GitHub directly instead of enumerating all PRs.