fix: add parallelism control over the e2e runner and add retries for rate limited requests

[rotem-cloud]

Add automatic retry mechanism for 429 rate limit errors
- Retry up to 3 times with exponential backoff (5s, 10s, 15s)
- Recreate command for each retry attempt
- Display retry progress to user

• Add automatic parallelism limit to plan/apply commands
  • Insert -parallelism=5 flag before positional arguments
  • Reduces concurrent API calls from default 10 to 5 (50% reduction)
  • Properly handles flag ordering to avoid Terraform errors
• Add 1-second delay after plan/apply operations
  • Provides pacing between test phases
  • Reduces burst traffic to Cloudflare API
    *update cloudflare-go 6.2.0->6.6.0

[vaishakdinesh]

@rotem-cloud do we still wanna see this go through?