feat: add dry-run mode to preview API calls and flyctl commands

[joshpollara]

Summary

• Adds dry_run provider attribute (also FLY_DRY_RUN env var) that intercepts all mutating operations and shows them as Terraform warnings
• REST API: non-GET requests return fake success and log method/URL/body
• flyctl: RunMut/RunJSONMut methods return fake success and log the full command line
• Read operations pass through normally so Terraform can build an accurate plan

Example output

Warning: Dry Run [1]
  POST https://api.machines.dev/v1/apps/my-app/machines/abc123/lease body={"ttl":60}

Warning: Dry Run [2]
  POST https://api.machines.dev/v1/apps/my-app/machines/abc123 body={...}

Warning: Dry Run [3]
  /usr/local/bin/flyctl ips allocate-v6 -a my-app

Test plan

• Tested REST API dry-run (machine update) — shows lease/update/release calls as warnings, clean apply
• Tested flyctl dry-run (IP create) — shows allocate command as warning, clean apply
• Unit tests pass (make test)
• Build succeeds (make build)

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