refactor: extract Lightsail operation helpers to eliminate CLI/REST branching duplication (#2468)

The AWS module had CLI-vs-REST branching duplicated in ensureSshKey (2x),
createInstance (4x), and waitForInstance (2x). Extracted 4 private helpers
(lightsailGetKeyPair, lightsailImportKeyPair, lightsailCreateInstances,
lightsailGetInstance) so each consumer is a single linear flow. A bug fix
in one mode can no longer be missed in the other.

Agent: complexity-hunter

Co-authored-by: B <6723574+louisgv@users.noreply.github.com>
Co-authored-by: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: 3 people