fix: idempotent lifecycle ops

[DominikPildner]

Description

This PR brings resource lifecycle operations in line with Terraform plugin framework best practices (see Read and Delete guidelines):

1. Read with missing ID: When Read is called before Create (e.g. by tools that observe existing infrastructure), resource IDs that are server-assigned will be empty. Previously this caused API errors from requests with empty IDs. Now the resource is gracefully removed from state instead.

2. Delete of already-deleted resource: When a resource has already been deleted outside of Terraform, Delete now handles the not-found response gracefully instead of returning an error.

Checklist

• Issue was linked above
• Code format was applied: make fmt
• Examples were added / adjusted (see examples/ directory)
• Docs are up-to-date: make generate-docs (will be checked by CI)
• Unit tests got implemented or updated
• Acceptance tests got implemented or updated (see e.g. here)
• Unit tests are passing: make test (will be checked by CI)
• No linter issues: make lint (will be checked by CI)