fix(storage): rate limit bucket deletion in cleanup

[abhinavgautam01]

Fixes #5219

Problem

When there are many stale buckets in integration tests, the cleanup process
was deleting them in parallel and exceeding GCP's Storage API rate limit
(approximately one request every two seconds).

Solution

Serialize bucket deletion by removing parallel spawning (tokio::spawn and
join_all) and instead delete buckets sequentially with a 2-second delay
between each deletion to respect the API rate limit.

Changes

• Changed cleanup_stale_buckets() to delete buckets sequentially
• Added 2-second delay between bucket deletions
• Maintains proper error handling for each deletion attempt
• Delay is not applied after the last bucket (no unnecessary wait)

Testing

This change should prevent rate limit errors during stale bucket cleanup
in integration tests.

[gemini-code-assist]

Code Review

This pull request modifies the cleanup_stale_buckets function in the storage examples to serialize bucket deletions. The implementation replaces concurrent task spawning with a sequential loop that introduces a 2-second delay between deletions to comply with GCP rate limits. Feedback was provided to replace println! calls with the tracing crate to align with the repository's structured logging standards.