Cleaner boundary between CLI and SDK usage

[klemen1999]

Purpose

This PR introduces more clear split between CLI and SDK usage on the architectural level. Longterm this ensuers cleaner code and easier maintainability

Split service functions in models, variants, and instances into:

• pure SDK functions that return typed objects / raise exceptions
• thin CLI wrappers that handle printing and terminal exits
• Added typed not-found handling via ResourceNotFoundError
• Added central CLI exception translation through run_cli()
• Removed CLI-only concerns like silent / field from SDK-facing service functions
• Updated convert exported-instance resolution to use SDK-safe polling and handle eventual consistency correctly
• Added focused regression tests for the SDK/CLI boundary and exported-instance resolution

Specification

None / not applicable

Dependencies & Potential Impact

None / not applicable

Deployment Plan

None / not applicable

Testing & Validation

None / not applicable

AI Usage

Assisted-by: Codex

Submitted code was reviewed by a human: YES

The author is taking the responsibility for the contribution: YES