Fix discovery: check resources of api_version exist

[amadev]

What do these changes do?

This change adds an additional check that resource exists in context._discovered_resources.

Description

Detailed info in the issue.

Issues/PRs

issue: #613

Type of changes

• Bug fix (non-breaking change which fixes an issue)

Checklist

• The code addresses only the mentioned problem, and this problem only
• I think the code is well written
• Unit tests for the changes exist
• Documentation reflects the changes
• If you provide code modification, please add yourself to CONTRIBUTORS.txt

[nolar]

The tests were failing for the example 10 — a known issue. It was fixed in 0.28.x in #619 (back-ported from the main branch from #608). The failure is not related to this PR, so it should be fine. I have double-checked in a follow-up PR #618 — the tests are fine with both these PRs merged.